You can't, Atheon will eventually teleport you and since there will be no one left in there you will die. So the minimum is 2. The only class that can solo Atheon is the Warlock class with the Sunsinger sub-class, but it's pure madness and demands getting the correct timing to kill yourself and resurrect every time for quite a while since you will have to kill him without the times vengeance buff. It honestly doesn't worth the trouble unless you're really trying to push yourself to the brink of insanity. Sorry for replying in English, but even though I can understand Spanish I suck at writing in it.