It's barely possible. You could try to solo the Templar, but you'll be dealing with enrage mode, and taking forever. Get past the Templar, then you got atheon to deal with. Atheon is gonna take some practice, but you'll have to do some hardcore parkour to get out of the intended playing area before atheon teleports you. Once teleported, it's a wipe. Basically, you have to go to the left and climb a wall. Once on top, atheon won't teleport you and you can spend all the ammo you want on atheon. It'll take forever, but with some vision of confluence and a shitton of time and you'll finish atheon. (Plus, don't worry about running out of ammo, because once you're out the game occasionally gives ammo. Usually primary.) Edit: forgot gatekeeper. It's physically impossible to do it solo. In the end, just try for solo Templar and atheon.