Defeating him in Corellia is the easy part (seriously)
When you fight him again in Korriban, if you use Khem Val (as many will), make sure you interrupt him every chance you have, and pay attention to draw Khem away from Thanaton if required (Ctrl+2 puts him on passive, moving Khem next to you, Ctrl+1 to make him attack again)
Myself, I was level 49, all upgrades from Corellia commendations - not an easy fight by any means. In case anyone cares - I leveled This Sith Sorcerer, a Sith Warrior, an Imperial Agent (Marksman) and just last night, a Trooper (Commando). The fight against Thanaton was *by far* the hardest until now.