I'm running with ~40k attack power (max level weapons, eidolons, and kamihime) and ~+94% water assault skill. 0% +HP since I use a +20% eidolon. That's likely what you need to do this smoothly, at least without stacking DEF DWN debuffs. Honestly, I could get the assault skill higher, but the Gem Getcha/enhance process is tedious and I keep putting it off.
The requirement for full-auto Ultimate is probably similar. You need enough attack power to kill the boss before it gets the chance to do significant damage, potentially without the benefit of debuffs.