Assa (Abloy, key company i'm always reminded of xD): Like the zon, this one can have very high level shadow master or the other one. And well, you can maybe use an occasional high levle trap, or if playing like relaxed, take it easy, and use freezing attack skills .. frozen blades n phoenix strike, to support merc. I like that idea. High level burst of speed to quickly go from one place to the other. Kind of fun.
The necro: Again, lots of support utility skills there. Amp dmg, leech, decryptify, bone wall (even, to save merc from big onslaughts), Gollum (golem ^^) ... clay (cold) golem, big level, very big level, gets like insane amount of hit points and the ultimative tank for the merc. And slows big the monsters around him, even bosses! Like, 79% slow + decrypt = Even with boss dealing big percentage dommage to merc = Still very little, since he is only hit about 1 every 4 seconds........... One can skip skeetons and only focus on support n utility. I like that idea, a lot! Make it easier for him.
Last but not the least: Druid: Again (and again xD) many many support skills. First one, oak man sage. Very very high level = BIG life boost. Very helpful. Can switch too with big level heart of the oak man, if merc gets good gear, and if you opt to choose the grizzly route, he can have good tank n focus more on dealing damage ........ now you get TWO insane tanks dealing big damage, at least teh merc though, i always found the grizzly a bit lacking. He waits before one of his two attacks, which is very annoying. Either way, you can also use raven actually, blind opponent = They freeze n you make it easier for your friend. Hurricane is a bit of a meh in this one, but you can focus on it, just help merc by slowing monsters. N no stress. I like that idea. Obviously little point in doing conversions here (transformation). Since there is no damage deealing oneself, or at least not directly. That leads me to do another point in paladin..............