I got this incidental to rerunning a few levels for collectibles. I was much, much better at the core mechanic of filling with Spiritual Energy and then dropping a Seal on enemies so they’d drop it and, consequently, being able to fuse with Monty more often. Don’t forget that a fused
attack will do the same thing as a Seal.
If you just need to know how to beat this boss fight go ahead and check my guide for it.
Honestly I was just trying to kill the bosses fast and move on (Normal mode). I think I had 5 bars of energy going in. Basically I fused with Monty around the time they had 60% life left and just burned them down (harvesting energy with the Shotgun attack as I went). If you feel like you’re not going to make it in time it’s better to die so you can try again without running the whole level.
I used Azia with all upgrades mostly because I’m best with it and there’s two of them so I could get mileage out of Spirit Knives having pierce and hitting both with a Seal at once. I stayed non-fused during their pillars phase (they only had one) because I wanted all my fuse for extra damage and immunity to knock back. You do have to play aggressively and take some damage but I think I actually cleared them in about 60 seconds and I’d forgotten this achievement existed.
It might be worth setting a 80-90 second timer on your phone to beep at you so you know if you should finish the fight or die and retry, but after you’ve made it clear through the game this shouldn’t seem too hard.
I would imagine once you’ve cleared the game, whatever weapon you are best at doing the rotation with would be the one you should try.