It's easy all you do is disable them and finish them I never lost a match using this method.
The main way I do it is:
If they have 2 weapons go for the legs
If they have a two handed weapon go for their arm
If they have a spear and shield go for the head
If they shield bash I recommend using double attack because you have more chance of hitting.
Or if they have low defence on their head or body target that for a quick kill.
The best way to disable them is using the power attack it does more damage then most specials and it doesn't use up that much MP I highly recommend it.
Another tip is too but slaves early on and build them up with you it saves money buying them in later places and you can hand down any weapons/armour you don't want anymore.