I think you are referring to the battle that has the 2 bosses, one that puts your health at 1 and the other that kills things below 35% or similar. The other 4 creatures in that battle are random, so sometimes it’s a bit harder and sometimes a bit easier.
I like to use a Nether or Death creature to use the death spell “Pestilence” (Gives all enemies blight and mend) then use a Nether or Life to cast “Divinity” (Resurrects and heals all creatures, then removes all buffs.) Because of the order Divinity does things in, if enemies have Blight it ends up hurting them. This only works if they would heal at all, so they have to be damaged at least somewhat. Even without Divinity hurting them though, it will still get all your creatures back to max health before the 2nd bosses turn. The Sense of Purpose ability (heal all your creatures for 25% health) is also helpful in that fight. Hell’s Protection ability might also help. (Your creatures take 1% less damage for every 1% health they are missing.)
Spoiler wise:[spoiler]When you get to level 60 you will unlock the Warlord, and then you can send your party on “Explore Land” missions to gain more levels. It takes a ritual, so you either need a 2nd party or lots of shards to use. You can easily over-level your creatures that way, and then that particular fight becomes much easier since you are more likely to kill at least one boss before their next turn.[/spoiler]