The more commonly known techniques are described in the topic
Chasing Josie... (minor spoilers). The topic is about Josephine, but the tips should work for any strong opponent.
Something I discovered recently is that Hell Spikes can be cast repeatedly fairly quickly. It is not as fast casting as Meteorstrike, but can do a lot more damage. Meteorstrike has a bit of a stunning effect, though, and is fairly effective even at level 1; Hell Spikes needs to be at a high level for best effect.
Survivor's Curse cast repeatedly (even just level 1) followed by any elemental attack works well.
Blind + Hell Spikes is quite effective against single opponents. With groups, it is probably easier to skip using Blind.
Any character can use a hit and run tactic with a bow, or Deadly Disks, etc. Burning Wall can be used either to slow down (and damage) a following opponent, or to surround an opponent (or group) to do even more damage.
If an opponent summons a creature, you can cast Aura of Command on that creature to have it fight for you. Some summoned creatures are relatively weak and will not last long, but others can do a fair bit of damage.
A warrior can drink a shadow potion, stand next to an opponent and use the spin attack (clicking on the ground somewhere) to do damage but not turn visible. This is almost a cheat, and will make any fight easy (and boring).
