Any boss creature from Darkmoor comes to mind as they have a large amount of health and great spell variety. Mostly, the creatures that are astral schools like Sir Blackwater and Spirit of Darkmoor. Also, you might opt for other creatures like Shadowbones (not that difficult to obtain) and Shadowwock. On the other hand, if you need powerful creatures that are extremely easy to obtain, won't take much time and can make some damage, go for the myth conscript minions you can find lurking around in Alkali Barrows in Mirage.
