Thymesia - YOU SHALL NOT PASS!!! Trophy/Achievement Guide
26Let me start by saying, this is hands down the most difficult trophy/achievement in Thymesia. Even if using a potion requires a bit of skill, every boss can be replayed and bested with significantly more powerful characters than originally intended. This, well, this is the exact opposite. I found no way to do New Game+ or retain any gear or benefits, meaning you're either good enough to do it or you're not. Still, I suggest beating the game before attempting.
Thankfully, there are some things to mitigate the difficulty. First and foremost, making a copy of your save after killing the one enemy before this can be reloaded to retry this fight endlessly. You can also close out the game while he is in the finishing cutscene and reload that to revisit the fight. The only thing to keep in mind about the other method is it saves with zero health and whatever resources you had prior, forcing you to revisit the checkpoint spot anyway and I can't confirm it doesn't change the fight in some way that prevents the trophy/achievement from unlocking.
All this being said, the best tactic is really to get good at countering a few of his strikes and alternating between countering whatever you're confident you can do and dodging everything else. I used Feathers very sparingly, as they regenerated quite slowly, and would only use them if he is regenerating or you know you'll not hit him in time to prevent it. Beyond that, keep your range and he should largely alternate through two very easy to deal with moves that makes this a lot more doable.