Xyrella + Cariel + Samuro makes for a solid baseline if you upgrade their abilities.
Xyrella should focus on Blinding Luminance and the Equipment that buffs it. It’s one of the best PvE abilities.
Cariel wants her Taunt at a level where it lasts (at least) 2 turns. Her +attack while Taunted gear works well here.
Samuro using his Burning Blade equipment and Double Strike will synergize well with Xyrella using Blinding Luminance on his target, and the combo should allow you to either heavily damage or outright remove an enemy.
Beyond those three, you should probably just bring reasonably upgraded backups for each of their roles. Level 1 abilities are not going to cut it in Winterspring or BRM.
You should note that several of the elementals in the zone have abilities that get countered by Fire damage, so if you have a merc with a Fire AoE they’re going to be a good option in those encounters.