If you have two controllers this is the fastest and easiest method for clearing a round while taking no damage, without the assistance of another person. It can also be done with a higher level friend instead of the two controller method.
You will need a higher level character and high level NPC allies—20+ is fine, but the higher the better—in the room of a newly created character, or one that hasn’t opened up the B Routes yet.
Start a game with the low level character as Player 1, then select the higher level character as Player 2. After that bring two of the highest level allies you have with you. They will help you kill things quickly and also guard the low level character.
Now go into the first stage of the game, the Ancient Temple Ruins. Use the high level character to clear the way. The NPC’s will mostly stay back with your AFK character which will help guard it. If you see anything spawn from the left side of the screen quickly dispatch it first.
Skip all of the optional areas and keep continuing to the right. Once you reach the spike room use the low level character and safely guide it through; this is the only time you will control it in the run. Once you reach the Harpy just kill it with your biggest attacks as fast as possible. It should die in a matter of seconds without it even being able to get off an attack.
If you were successful the trophy will unlock after it dies.
If you only have one controller you can also do it like this, but it’s riskier:
On Normal difficulty beat Old Capital to unlock the “Beastmaster” quest, which puts sabretooth tigers at the start of the stage. Load up Old Capital by yourself, with no allies whatsoever, and kill the enemies without being hit. Mount the sabretooth tiger and go through the stage killing everything. If you get hit while mounted it won’t negate the trophy, but try to avoid as much damage as possible so your tiger doesn’t die.
Skip the optional rooms and make your way to the underground area, where you will run into a blue Dragonkin/raptor looking enemy. Attack it and when it's on the ground swap out for it because your Sabretooth will have taken a bunch of damage by now. Use it to fight the boss and again, avoid as much damage as you can. If you do it right you can kill the boss without ever being dismounted.
If for whatever reason you get dismounted try your best to avoid damage and kill off the boss.