I would go back and kill him, he has a *really* good item that you will want if you are a good aligned party.

That mage who was really easy is actually about 3 levels higher than the one with Draug Fea. The only difference is that being alone you can concentrate on him (something you should do with all mages whether or not they have a party with them).

Frankly if you managed to do that dungeon, I don't quite understand how you could find Draug Fea hard.

As for killing Firkraag I would wait on that, both because if you're playing a paladin killing Firkraag will be one of your quests later and killing him now will mess that up, and because it's a hard fight if you don't like exploiting the flaws in the AI, with a high chance of a wipe if you don't have the right spells.