How did you manage? The fights are so hard!
the trick is to make sure that you're always leveling to the highest tier of gear available, which is fairly easy once you get access to the stickers—and on Devil Clock you get lots of stickers! otherwise, 5 Battle Energy and the consumable that gives you nuclear element are both extremely powerful, so much so that the game turns the latter one off for most boss fights.
for strategy, using Bleed on the last board, Sam set to Splash Heal, and using debuffs/taunt/Life on the preceding boards while you wait for Sword of Fate was my strategy. Bleed does ridiculous amounts of damage, so I had it on both Val and Grace, with Smite on board one literally just for Smite Only challenges. I used Angela exclusively to generate Ravage meter.
whatever your last board is, is what you'll spend the most time in naturally, so it's good to have it be consistent exploit damage + group healing. whatever your second board is should be buffs and heals, because until you get a Fast passive trinket you should always immediately advance to the second board. I put "situational powerhouse" moves on board one, since you use it the least; there's a combo with Fast passive skills and a tier IX trinket to get powerful staggers easily, but I think it's more fun to figure that out on your own.