I've rolled a couple of priests in the past, and recently I transferred one to play with a friend (He's 70, I'm 23 :P ). All my priests are stuck in the early 20s simply because I have trouble killing things my level or above.
It's namely, I think, my cast rotation. I start off with Bubble, then Holy Fire, SW:P, Mind Flay, Mind Blast, and then wand down if he's low. if not low, I do another Mind Blast and wand.
Even though I have really good gear for my level (A ton of blue cloth form SFK), I'm terribly mana inefficent, even with spirit tap, and I'm mostly in int/spi gear.
Any help would be greatly appreciated.