There's a fair degree of knowing how to play a caster class, and knowing that you're actually okay not being at full health all the time. Don't exhaust your spells, you shouldn't need them most of the time, and when you do, choose wisely. Being able to change a cleric and wizard's prepared spells on the fly out of combat is more than enough to change the tide of battle.
Cantrips seem weak, but they're really quite powerful in 5e. Even more so here because there's environmental effects going on. Blood + Ray of Frost will often result in Prone from the ice patch, which then means an automatic critical next attack (which could even be immediately after the cantrip causes them to fall). Three party members in a row can take down very strong enemies this way.
I've been playing a wizard main with Shadowheart in the party but I've only been to camp three times in 15 hours. Quite a few of those hours were running around with less than 5 hp left on a few characters. Using Help on party members up a lot though, hah.
Also, eat food to conserve cleric spells.
It's the utility spells I end up never using because I have no idea if they'll work or not. Detect thoughts hasn't worked a single time I thought it would.