Thanaton was EASY for me.
Even though I screwed up with my Assassin's specializations (they were all over the place), I had a pretty easy time defeating Thanaton in the Dark Council chambers.
I brought Talos with me, and popped a Willpower stim right before.
I finished the fight with nearly all my health, and all of Talos' health as well.
You want to save Jolt and Spike to interrupt his healing power, and then simply back away during any AoE attack like death field, EXCEPT Lightning Storm, as he constantly uses that during the first half of the battle, and if you have good armor, it barely scratches you, so just fight through it.