Impossible. The game forces you to have at least 4 people on Atheon (or you'll die in the portal) Unless you could kill him before he teleported you (you can't anymore) you'll die.
Edit: Minimum is infact 2. Self res warlock can bypass the teleportation.
English
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You can solo Atheon in two ways with only 2 classes, Susnlinger or defender, Suns use self res before teleportation and defender climb to the cheese spot.
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You can two man with a self rez warlock. Just die before teleport and self rez after to open the gate. 2 is minimum.
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Someone has soloed it, of course he glitches out times vengeance, but he still soloed it
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Ok, wasn't aware. Thanks!