So what? Move on to the hard campaign and come back to finish 3* on normal later, when your second champion has better gear. Those rewards aren't going anywhere. 

what is vamp gear?

I don't even know what to say about that.... If you don't know then you are a new gamer indeed.

In his defence, it's not called "vamp". It's lifesteal gear.

i think it depends on the language, but vamp = lifesteal.

get him to 60 and he'll walk through it solo.  in the meantime, focus on farming 12-3 or working on the hard campaigns, you can circle back around when your champs are a bit stronger.

level 60 makes a masssssssive difference.

What's the problem? Does ur warpriest or Kael not have enough masteries? Can u show me Kael's gear or at least show me ur roster? That would let me help u much more

If you want to 2 champ with Kael at lvl 40 - you need much better gear.  Otherwise, as people suggested above - move on - and then come back when Kael is lvl 50/60.

early game  a star helps, until your champs get to 60 and in better gear 1 star still a win.

key to early game get your kael to 60 fast as you can and fully mastered, you see a massive difference once you do.

share your champ roster we may be able to suggest a different match compilation to try