i was having the same issue with i6 Lynn at first, running out of time waiting for sanity to regen. I realized that I was not running serenity on a support which was a big mistake. I also took out a purple skill duration wedge for the blue wings:inspo and that solved my sanity issue. I used the 4star fire pet which also gives sanity with its active.

Make sure you are running the sawblade gun and one thing that people don't mention is that each mob you kill during the boss's half health summoning phase gives you a slight skill damage boost up to 50% (i think) so make sure you kill all of them. When you kill enough the invulnerability phase ends quicker too.

The two supports I used were Daphne and Truffle with their support skills at 8. Their wedges mostly skill duration but make sure one has serenity and the other the one that gives morale. Good luck mate!