I went through some quests with Hastral last night, which got him to level 63. He has some trouble with soloing group quests, but 2 man quests aren't an issue. Killing regular mobs, though, is a breeze. I have no problems fighting several at once. I throw DOTs at one mob, throw a haunt if it has high health or I'm low on mine, and move one to the next one while DOTs kill the previous. If I'm low on mana i just use Dark Pact or Life Tap.
I haven't done this from the start, but i plan to take a screen shot at the beginning of each level for every one of my characters. You can find them in each of my characters albums in the sidebar.