Just like Elden Ring, the strongest boss is just a copy of me. That Mimic was fucking PEAK. What a great boss to end the game with.
Her AI is cheeseable, this phase is basically a war of attrition and you want to have as much windows to out damage her. Spam arrows if she runs away, spam slash if she tries to get close, then repeat first step.
2nd phase opener, hide below the platforms so you don't get hit by the slug balls, repeat first strategy
3rd phase, go to the right most area of the map when she summons the nuntipedes, then at the center of the two on the right, then right most part of the map again. You want to be either behind the one closest to you, or as far away from their range as possible.
Repeat first step, congrats.