Shaco can solo the rift herald at level 2, theoretically. In practice, it's hard as fuck.
More realistically, since it spawns at 4 minutes, you can solo it at level 3 if you back after hitting 3 and then setup for about 50 seconds. If you setup 3 boxes separate from each other, and juggle aggro on to them, placing a fourth box on top of the third box as it's being aggro'd, all the while attacking the eye from behind (which is only open if there's 2 visible attackers), so long as you get at least 2 hits on the eye per box for 3 of the 4 boxes, you'll solo the rift herald with maybe half health taken.
It takes a little practice, so don't be surprised if you die on your first couple attempts. If you don't time the last box properly, it won't maintain aggro against the herald and you'll take a lot of damage trying to make the box take aggro again, and you might miss an eye hit if you auto just after it kills a box. If you get the pattern down, it should be pretty uniform.