I played all the way on hard, just finished the game and here's what I did against Revenants.. (I completed the quests where you have to kill those with buddies in elven lands as well as the other that has you kill…like 6 revenants I believe across the land).
As some people mentioned before, having a tank keep aggro as much as possible is a must. Cone of cold to freeze him works quite fine to give your guys a sec to quaff some potions.. Force field is also extremly useful against just about any foe, you may not damage them, but it allows you to recover and your spells that are on timer to reset… Before Force field / waking nightmare and using crowd control better I had a bit of trouble with the revenants that had buddies, but after that it was relatively easy.
The one annoying part I found was that the revenant aggroed way too often on Morrigan for whatever reason heh, and kept casting pull. So just being away doesnt work as he brings you all the way to him. There's two tricks I found when he aggroed Morrigan too badly, either have her run around and cone of cold him every now and then.. And hope for tank to get aggro back
, or you can place whoever is being pulled behind an object. You'll still get pulled but you'll slam into the obstacle and remain there rather than be pulled all the way to him, after that he usually switches aggro back to the tank.
I must have fought ash wraiths at some point although I can't say precisely where. But I probably used crushing prison there, thats one great spell....hrms...and maybe petrify too if Im thinking of the right mob.
As for drakes and high dragons and such… Freezing works superbly on any type of dragon in the game to give you some time to heal yourself or damage them. If you use some lesser/greater warmth salve you'll also be able to withstand any fire special attacks they might throw at you with ease. The drakes in quite a few places came with groups of mobs, there I often usead sleep + waking nightmare. Its fun to get them all stunned/confused and fighting amongst themselve
.. Also I believe regular drakes can be stunned by a rogue.
In all the hard fights timing is also important
Timing your cone of cold or whatever ability you have that may interrupt them is way more useful if you do it when they're actually casting a spell/ability.