It's easy to do the mana regen thing with one BD. I'm also up to level 43 and have been using similar tactics as DGD.
However, at the start of a battle, i usually cast Demon Portal, then when they emerge, cast Stone Skin and Mana Spring on them. Then i let them take a beating. If there are lots of ranged units or Shamans, then a level 3 Target on them also helps.
So while they're getting the crap beaten out of them, your BD is free to soar around and open chests etc. Towards the end of the battle, you can use the BD's Fire Breath on the stacks around the demon to get your range back up if you have the Paladin's Holy skill maxed.
