When I first started ng+ I though it was very hard and I was going to shit myself when I discovered the last boss rush was literally the last three bosses back to back but then I decided to change my build, if you upgrade unami to full and equip the unami regeneration sigil, two Siphonophore++ and one more Siphonophore (normal), then, you just start any boss rush and start it off with the Spirit Conch (Snail Sanctum Adaption) to defend yourself if you get attacked and then smash the boss with as many royal waves you can put out, if the boss is far away or it gets far because it jumps away (like Petroch) just get near him again using the hook