Paladin + 4 priests

[Apatheist]

Renew doesn't stack.

Holy nova costs from 185 mana at rank 1 to 750 mana at rank 6 and does negligible damage (7.14% coefficient.)

In your 20's-30's you'll be OOM in 3 GCD's and you definitely won't clear dungeons spamming holy nova.

There's actually a video of a guy farming with holy nova here. His talent build is entirely based on reducing holy nova cost and he's stacked intellect on all of his gear. He can cast holy nova 10 times before he's OOM -- hitting for around 200 damage per cast. That's 2000 damage plus crits before you're dry. To put that in context, the ogres in Dire Maul have around 14,000HP.