ACE difficulty is the hardest difficulty within the game and infamous if you have played any of the other Ace Combat Games. Functionally it is the same as all the other difficulties, in terms of gameplay and S rank requirements, but where it differs is that if you take even a small amount of damage you'll be as good as dead (1 missile is insta-kill and around 5 machine gun hits on you is certain death). Also important to note: wingmen kills also add to your score so don't think you need to kill hog, set them on attack 
or disperse 
with special weapons allowed 
to maximize your effectiveness.
If you are having trouble completing some missions, unlock and save up to buy the ADFX-01 Falken. It costs 897,000 credits so it takes a lot to pay for (around 1 and a half to 2 full playthroughs of getting all S ranks will get you that much). It may seem like a normal airplane but it's special weapon is a Tactical Laser (TLS) that will basically insta-kill anything it touches that is unfortunate enough to be in front of your plane and under 10,000 m. away. This weapon will trivialize some of the more tough missions to complete without taking much damage (like White Bird Part II) if used correctly. To unlock it you must destroy the Hangars in five separate missions. Once you have destroyed one your AWACS will say "mechanical parts discovered in destroyed enemy facility. Let's ask the ground forces to retrieve them later". You will also get a notification saying you have acquired a part of an enemy experimental plane on the mission stats screen. After destroying all 5, the Falken will be available for purchase.
Falken Locations:
- Mission 12A: "Powder Keg" - Follow the river you start near to the point where it forks into different paths. Follow it directly to the east and it will be near the boundary. Once you get near it, it will appear on radar as well as a MiG-29A
- Mission 12B: "Four Horsemen" - After penetrating the radar network it will be within the enemy base, directly next to the SAM sitting by itself. It will appear on radar after destroying a few of the ground targets but you can destroy it witch machine guns before it appears on radar
- Mission 16A: "Desert Arrow" - In between the enemy's field HQ and the air strip. When flying in between the two it will appear on radar sitting all by itself.
- Mission 16B: "Desert Lightning" - In between the enemy's airfield and oil refinery. When flying between the the two it will appear on radar sitting alone near the bank of the river
- Mission 27: "Aces" - Marked from the start sitting next to the two tanks stationed on the far bridge, quite difficult to miss.
While the Falken is not required for S ranks, it makes missions much quicker and easier, however the below scores are what's required for the S rank in each mission and the tips are assuming you are using standard aircraft. Also aim for the named aircraft in every mission they are available (see GRAND FALCON for more details) as they can be treated as free points which definitely helps in obtaining S ranks.
Mission 1, "Shorebirds": 1,700 score
All you need to do is shoot down two waves of fighters to achieve the necessary points.
Mission 2, "Open War:": 1,250 score
Basically unmissable, the mission is to shoot down all air targets so you will obtain this regardless.
Mission 3, "Narrow Margin": 9,400 score
Destroying all planes in the first phase will net you around 3,800 to 4,080 point max. In the second phase, destroy the MIR-2000D, the Nimrods, and the two F-4E's before turning your focus to the ships. If you are still way under the point limit, leave one ship (not missile boats as they are non-critical) and wait for the second flight of F-4E Phantoms to make it to you for extra points.
Mission 4, "First Flight": 10,000 score
In all three phases, make sure to destroy all escort fighters before destroying the bombers (TGT) to avoid advancing the phase too quickly, especially in the third phase, to get enough points for this S rank.
Mission 5, "Rendezvous": Complete in under 9 min 30 sec
Use a plane with high speed in order to take out all targets within the time limit as they are all mission critical. XMAA's definitely help with some of the further away targets however they are not needed as long as your airplane is fast enough.
Mission 6: "White Bird (Part I)" - 10,000 score
In the first part of this mission, aim for many of the C-130's as each of them nets 800 points upon destruction, but do not forget to destroy the tanks too or else you will fail the mission. In the second phase, aim for the B-2A's while shooting down some of the missiles as each missile is only 100 points, while the bombers are worth way more. If you need more points, destroy some fighters straggling around before the STO launches.
Mission 7: "Front Line" - 17,000 score
The ships during the first half of the mission will be the main source of your early points. If you are far away from the point total when you are battling the Scinfaxi, aim mainly for the aircraft it launches as well as the SAMs and AA guns on it for extra points as you do not get credit for destroying them when the sub goes down unlike standard ships.
Mission 8: "Handful of Hope" - 3,600 score
After guiding the transport plane through the radar network, you need to destroy around 10 planes, which shouldn't be that difficult to accomplish. If you manage less than this many kills, the windmills also add to your score and help you to the S rank.
Mission 9: "Lit Fuse" - 8,000 score
Your main target in this mission is ground targets, which do not give many points upon destruction so your main source of points will be from aircraft. Make sure to shoot down any F-4X's, A-10A's and TND GR-1's in between ground targets becoming available to get enough points.
Mission 10: "Blind Spot" - 13,000 score
To reach S rank score here all you will need to do is shoot down the fighter escorts before downing all of the jammers and transports.
Mission 11A: "Chain Reaction" - Complete in under 8 min
Choose a high-speed plane with favourable special weaponry as they will save you time instead of just using missiles and guns. Shoot down the fighters as quickly as possible then turn and fly back to the airport. Focus solely on the C-5’s on the ground as well as the tanks. After all ground targets were destroyed, turn your attention to the new planes that spawned in, hopefully your wingmen hurt/destroyed some of them.
Mission 11B: "Reprisal" - Complete in under 10 min
Switch to special weapons immediately and right away aim your UGB circle on the most yellow part of the smoke trails on the ground. You are going for as many as you can in one run so don’t be afraid to fly slowly. After this you will need to sit idly for about 5 minutes while the car chase happens, after which you will need to down 8 helicopters with your machine guns to complete the mission, so stay near the car so you can get to them quickly.
Mission 12A: "Powder Keg" - 10,000 score
As you are going through the level, pick off any and every non target and aircraft whilst also taking out all but one target. If you still need more points with only one target remaining, either attack the Falken hangar or try to destroy some of the SAMs in the jungle with your machine guns (they are about 100 a piece).
Mission 12B: "Four Horsemen" - 15,000 score
After penetrating the radar network, destroy all of the transport planes and a few of the marked targets to get all of the ground targets on radar. Now destroy all aircraft and non-targets before mopping up the remaining targets.
Mission 13: "Demons of Razgriz" - 6,700 score
This mission is tricky because if you use certain special weapons (GPB for example) you can damage it too quickly for you to get the necessary points. Destroy one/two subs on your way to the Hrimfaxi and the supply sub after damaging it. Always shoot for the UAV’s and subs that spawn. The sub itself is worth 2,000 points so when you are within range where that will get you S rank, turn most of you attention to the Hrimfaxi and its AA defenses; it will stop launching planes once you've made it reach around half health.
Mission 14: "Ice Cage" - Complete in under 11 min
Fly completely straight in the direction you start the mission in to find your first group of fighters. Then head in that same direction after downing them to find the next flight of five, after which the final flight of five fighters will be at the bottom of the map in the center (they will briefly appear on radar). After this try to mop up the AA defenses as quickly as possible then shadow the helicopter. Once it gets far enough, destroy the 6 new targets and then just wait out the dialogue.
Mission 15: "White Noise" - 8,000 score
Destroy the first group of fighters and then turn all of your focus to finding Nagase. Destroy the jammer that spawns in when you get close to her then actually find her ASAP. Then destroy all of the AA defenses near sea goblin and then just start destroying any fighters in the area. By the time the helicopter makes it to her, you should have well over the required amount of points.
Mission 16A: "Desert Arrow" - 21,500 score
It is easy to get above the S rank score before the mission update if you know how the enemy will attack. They attack in a zig-zag pattern (first the North group, then the South) so respond in that order. Destroy as many ground targets as possible in the airstrip and field HQ to boost your score but don't waste all you ammo on them. Destroy the fighters flying with the AWACS if you are still in need of extra points.
Mission 16B: "Desert Lightning" - 24,000 score
It is easy to get above the S rank score before the mission update if you know how the enemy will attack. They attack in a zig-zag pattern (first the North group, then the South) so respond in that order. Destroy as many ground targets as possible in the airstrip and oil refinery to boost your score but don't waste all you ammo on them. There are two fighters flying with the battleship formation so pick them off as well if you need more points.
Mission 17: "Journey Home" - 23,000 score
The goal here is for you and your wingmen to destroy as many aircraft as possible as quickly as possible, as this is largely a time based mission. You will have nearly enough points by the time the stealth fighters spawn in if you destroyed all other planes beforehand. If you have under 20,000 after the cutscene, consider restarting as this phase is very short and you will need to hurry to get enough points here.
Mission 18: "Fortress" - 11,000 score
Destroying all of the ground targets gets you nearly enough score for S rank so just pick off some of the fighters while hitting the ground forces and you will get this easily.
Mission 18+: "8492" - 11,800 score
To get S rank here, you simply need to shoot down all planes in the area. This is no easy task but just ignore your wingmen's suggestions to flee the area.
Mission 19: "Final Option" - Complete the mission, given automatically
Mission 20: "Ancient Walls" - 14,000 score
During this mission take out the tanks and AA defenses as soon as they spawn as they spawn infinitely and are a good source of points, but more will not spawn in unless you destroy the ones already there. Down all fighters as they spawn for extra points. Hold off destroying the wall if you are seriously under the limit and pick off some more fighters but know that stealth fighters will spawn after taking it down so you don’t need to reach that score before taking down the wall.
Mission 21: "Solitaire" - Complete in under 6 min
Choose a plane with high speed and manoeuvrability and fly at nearly top speed around the radar circles at low altitude. Once through, use your special to take a picture with all 3 marked targets in one frame. After that, get both planes that show up in one picture. If done correctly the game will say mission update and then immediately fly to the dotted line due East, no need to wait for Pops to say the picture was good first.
Mission 22: "Closure" - 17,700 score
Destroy all non-target ground troops as well as aircraft en route to the mine entrance. Once at the base do much of the same, attack ground targets (more will appear the more you destroy) and aircraft. Once you have over the target number of points, then focus all fire on the mine entrance.
Mission 23: "Ghost of Razgriz" - Complete in under 15 min
Destroying the initial targets is no problem, you can do it in 4-6 minutes depending on the route you take. The main time sink here is the second phase, destroying the Ofnir fighters as their path takes many sharp turns and thus avoids many missiles you fired. Follow them until they fly mostly straight so a missile can hit them, or you can try with machine guns if you get close enough but this will probably take more time than using standard/special missiles to do the job.
Mission 24: "White Bird (Part II)" - Complete in under 7 min
This is one of the hardest S ranks in the game to pull off due to the strict time limit. The key to this mission is destroying all 5 five engines so do that as soon as they become available to save time. Use time between the engines becoming available to destroy pieces on the Arkbird as well as the UAV's, (the launchers are the first things targetable and if you destroy them it will stop launching UAV's). Obviously a plane with high speed is a must here.
Mission 25: "Heartbreak One" - Complete in under 15 min
Every time you are asked about a shortcut or long route, choose the shortcut to save on time. Every time a checkpoint (TGT) becomes available, destroy it ASAP. Doing this should give you enough time to dogfight with Grabacr without worrying about time. If you are having trouble, set you wingmen to cover 
and speed away. Once you are far enough away, turn around and you will get to shoot at Garbacr head on destroying one if using missiles, all if using XMAA's.
Mission 26: "Sea of Chaos" - Complete in under 9 min
The limited aircraft selection actually works in your favor here, choose the F/A 18C Hornet, mainly for its LASM special weapons. Destroy the carriers ASAP to limit planes spawning and then use LASM's to destroy ships further away while using machine guns and some missiles to eliminate closer ships. Try and save ammo and time for the final part of the mission fighting against Typhoons, SU-47's, and F/A-22A's, all of which are more manoeuvrable than your Hornet.
Mission 27: "Aces" - 15,000 score
Aim for the air targets mostly of all as some of the cutscenes showing you allies advancing can actually despawn some of the planes as well as any missiles you have fired if you clear out certain ground targets. Destroying all ground targets, non-target ground troops and aircraft should net you around 12,860 (more if you destroyed the infinitely spawning SU-47's to the north) before the tunnel is opened. The target in the tunnel is worth 2,000 points so you can use the SU-47's in the second half of the tunnel to get yourself over the point threshold. XMAA's help tremendously if you are severely lacking points as the second group of planes cannot be fully defeated without them.
Mission 27+: "The Unsung War" - Complete in under 20 min
Speed to the SOLG descent site ASAP to give yourself the most time to deal with the flight of 8 aces. Using XMAA's can take out a few of them immediately making your job easier so you don't need to dogfight all 8 X-02's. Destroying the SOLG is simple enough, just beware of parts shot off it as they can kill you if you hit them.
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or else you will skip these). To get this dialogue, you will need to complete any landing mission in under 40 seconds and refueling missions in under 20 seconds. The trick to refuelling missions is to line up with the back of the plane then speed there and slow down and don't move for 1-2 seconds once it says "Hold Position". For landing missions, speed toward the runway and once you get over the tip of the runway, hammer the brakes and try to stall right as you touch down on the ground. The missions that have these are:
