I think the Ampule dungeon is the best choice. You always want ampule (unless you litteraly swim in them). It's an easy and cheap way to boost your girls. Or you can farm main sorty mission with exp bonus to quickly level up and increase your stamina.
Then, Memory dungeons that gives petite oath heart are great.

For raid bosses, yeah. It's a bit of an issue. Just use one low level 2* or 3* to poke them. It will reduce loading time and combat time. Someone else will get the kill anyway. That faster you end the fight, the more chance you have to get another good one. Also, avoid raid bosses shared by friends. Some share them to friends only and no kill them.
Raid bosses out of raid bosses events are not worth it anyway. Low tier equipement seeds are beginners traps. Mid tier seeds are trash, high tier seeds are worth only for the abysmal chance of getting an EX ring (not even that worth it now). Only the new bosses with high tier seeds and low chance of aqua fragment deserve attention. But aqua fragment drop too rarely to make it significant.
Just focus on event equipements and don't take thoses raid bosses too seriously.