1) Lag them when they are not ready for a fight.
2) Attack from hiding (similar effect).
3) Have someone steal their wands.
4) Avoid fights when they are prepped, so that they stop prepping because they want a fight.
5) Sneaky tricks like choking them underwater and hoping their waterbreathing fails (requires you to get them to chase for some time.
6) Make sure you use weapons they don't know, and dual wield for most magi. Those grazes can add up fast.
7) Don't fight beyond the point of no return (e.g. if they are maladicting you).
8) Make use of healing. If you are not prepped, you can heal 100 hp a pop at a healer. If they are prepped, they can only heal a tiny fraction of that, so again, in a drawn out fight, your grazes will have more of an effect.

Generally though, the best tactic is to whack them when they are not expecting it. Even the toughest guys will often go down like that.

I'd say, "Bring a friend", but as Victoria those that brought friends tended to over-do it and suffer as a result. A pair makes quite a good number though.