Walkthrough of the Life campaign on Champion difficulty.

You need Resurrection for backup, and you had 5th level Life Magic, which is also resurrection. As I recall, I didn't have Resurrection from the blacksmith either – I found 1 potion in the cart. And I didn't have 5th level Life Magic either.
If you don't have Order magic, Nature magic will be just as good. And even just Life magic (but with 4 heroes) would probably be enough... But without a tank, there's no one to fight with Life magic in this situation.