Quickest way to do this is play through the first 6 levels on Normal, if you struggle on a level switch to casual. You get much bigger bonuses it seems for completing a level the first time than redoing it even with a higher score or difficulty. (I tested this with the Archer after playing through normally as a Mercenary and then on co-op as a Sorceress).
If you were hoping to get boosted this using an AFK by another person, I have some bad news, when we tested this in one level my co-op partner levelled up and I didn't.
Good luck, 10 levels isn't that long and you should have a feel by then which of the 3 you want to stick to for the long haul of the games other achievements.