Ok, main focus is to prevent party members from dying. So i feel like ClericC2 and PriestC3 along with Plague Doctor will be mandatory. That means we have only one class selection left. And since all priest abilities focused on physical defence, we will be lacking of magical. So maybe paladin for Resist Elements and heal increase or pardoner for Increase Magic Defense?
Any thoughts? Whats your ultimate survival build?
Where’s your Chaplain? 
My build would be cleric2-krivis3-pally-plague doctor.
Max out Aukuras. it and pally’s restoration stack, giving your group, and indeed, everyone nearby, a 500hp(plus natual regen) heal every 5 seconds in combat.
Plus, Paladin’s regen adds onto normal heal tiles as well.
And then plague doctor saves everyone from status effects and gives yet more healing.