Defeat the Dragon Boss with 5 Saurian Fighters alive.
This boss is weak against Water/Frost Damage. You can solo him starting around Level 15, but if you want to do this trophy quickly and burn him down, it’s recommended you wait until you reach Level 30.
To start the fight, you need to hit the egg that is on the platform. The main difference in the fight between going solo and with other people is that the Dragon’s HP is a little higher. It’s nothing too drastic, but it is noticeable.
As with the Worm Boss, there are two parts to this fight that will repeat until he is down.
Part 1: You cannot attack him during this part (he is immune to everything). The Dragon will fly around and spit fire eggs at each active player. You need to dodge them, line one of them up with the Dragon, and hit it into him. He will alternate between spitting one and three eggs, so just make sure you keep moving to dodge them.
Part 2: Once you hit him with one egg, he will come down, allowing you to hit him. BEFORE YOU HIT HIM, make sure you hit any remaining eggs off the platform. These spawn the Saurian Fighters and they are a pain to deal with during the fight, so hold off on spawning them until the end. When there are no more eggs, attack the Dragon. When he gets back up, you’ll rinse and repeat this until he is low on health (you want him to be about 3-4 hits away from death).
When he gets low enough, allow him to keep spitting eggs onto the platform until there are at least 6 eggs on the platform (though I recommend 1 or 2 extra for cushion). When there are enough on the platform, hit him with one. When he comes down, don’t hit him to kill him. You need him to stay alive so the fighters spawn.
When he flys back up, all the eggs will turn into Saurian Fighters. If there were at least 5 eggs, you will be able to get this trophy. Kite the fighters around while the Dragon spits eggs at you. Get one lined up and hit him with it (note this is much easier said than done as you will need to dodge and roll away from the fighters attacking you). Once the Dragon is down, finish him off.
If there were at least five Saurian Fighters alive, this trophy will unlock. Killing the Dragon does NOT automatically kill the fighters. You will still need to get rid of them yourself. If you don’t want to kill them, pick up all the loot that dropped and run through the portal that appears in the middle of the platform. It’ll transport you without you needing to kill the fighters. If you did all of this on your first encounter with the boss, Dragon Slayer will also unlock.
If you make a mistake, you can reset the boss (and the fight) by having all players go back down to the Landing Pad (either run or teleport through the map).
If doing this at Level 30, the Talents/Specs used for each class can be found in this post.
Below is a video showing this fight solo with all three classes, and one run on co-op, just in case you need more help.
Note: The Ranger in the video is level 28 with level 22 crafted gear. The Mage and the Warrior are both level 30 with level 26 crafted gear. Everything they're using/wearing can be crafted at the Workbench (Ranger), Altar (Mage), and Anvil (Warrior). This is by far not the best gear available in the game.