I tried a similar thing and it was a lot of fun! I gave mine a shield so that he could act as a tank as well (and did a reversed-role party by making my warrior the glass cannon). By the end he was enormously powerful and practically unkillable. It was really satisfying stunning, freezing and petrifying three people and then Death Punching a fourth :D
Sadly his damage never amounted to much because Death Punch's cooldown never got low enough to spam and it did less damage than a scythe hit for a whopping 9 AP. Only good for style points, really. He made up for it in status effects though - you have a 100% hit rate against enemies that can't move, so he would stun a couple people and then my stealthed glass cannon warrior would appear behind them and chop them to pieces with her scythe before slipping back into the shadows. I like to think of it as his own custom version of Death Punch ;)
My favorite spell was actually Explode - the AoE is fantastic and it only costs 3 AP, 2 with Elemental Affinity. The damage isn't spectacular but it's cheap, can be cast every round, provides easy healing with Fire Shield and is just fun to use. I imagine it'd be even better combined with a traditional poison-using mage. Still, his main contribution was status effects; I just like fire.
Elemental Affinity turned out to be a better pick than I expected. You tend to cast more spells than the usual mage because they're so cheap and you also run into more terrain than the usual mage, so it's easy to save an action point or two. If you can save one every round that's effectively 2 speed and 1 con! Between explode, haste and fire shield I saved a lot of AP. Also, blood is plentiful and counts for Witchcraft.
