At first I thought you would necessarily want a party of four for the extra HP, but since the enemy group scales with the size of your own, Jasper & Flame make an invincible combo if you can survive long enough to get the right upgrades.
Having only those two means no one is competing for the same mana color, and Jasper can keep Flame fed with blue until he becomes mostly self-sustainable. After a few burn-stacking and AOE upgrades, Flame can clear the board insanely fast while Jasper intervenes every so often to replenish mana (mainly by tripling the leeching on 1-blue enemies). Now I get what you meant by never having to end a turn! 155 overkill at level 20 killing 3 to 4 enemies per action feels pretty satisfying, except for how long it takes to resolve all the effects triggering and the constant screen shake :P
Quick bug: texts.room-loot-goal is missing when opening a chest.