It is possible to solo atheon still in 2 ways. 1. Warlock hide in middle platform (or anywhere else just this spot works best) and non stop grenade spam, right before he teleports you (use a timer to keep track) fall off, wait a bit and self res. 2. This way takes longer but all can do it. There's some parkour to do and basically jump out of his range left side of the map and jump when he teleports (somehow it blocks it) and just snipe until dead. I'll try to find the videos that taught me this if you'd like. I didn't find these myself, credit to video people who I'm still trying to find again.