Just found an OP build.

https://reddit.com/link/1r4f0a3/video/2t88cfb62fjg1/player

You can skip to 2:13 to see it in action.

Basically, my mage can reduce all their spell cost by 1, then it can reduce an ally's spell costs by 1 and finally give 4 mana, while spending only 1 mana, thanks to the spell cost reduction.
Then cleric can give a Divine Protection to one of the allies that gives 1 divine shield and 2 kinetic spikes and do so only for 1 mana, again thanks to the spell cost reduction. And so, I just spend all the mana for the Divine protection and make a Thief basically immune to all damage, except bleed and poison.

Yeah, you can shame me for save-scumming twice in this game, but in my defense, I was just trying to properly set up and test this build :<