From what i'm reading no 1 can understand why they are dying to the bosses. I think the main issue is this is the first challenge we've had that requires strategy and not just damaging the boss.

if the boss lands under 50% he is going to wreck your team generally 1-2 people will die unless you have multiple 6 stars and bar-element/shellga/focus on your units. The trick I used was slowly damaging the bosses to around 55% at this point a 6 star lightning can almost finish them by themselves. I know not everyone has one but you most likely have a friend. Kain is a common unit almost everyone should have max him out asap and his dragon dive with a lightning and what ever other damage units you bring should be enough to finish each of the 4 fiends off.

For golbez same thing don't knock him lower than 50% until you're ready to kill him or be damn close to equip someone with anti paralyze so you can chain cure everyone with items and it should be GG.