you can probably do everything on the solo artist challenge up through solo disciple, maybe even some of the howling stones named as long as they are mezzable (if 53 or lower u should be good). getting something like orb of tishan or journeymans walking stick is really important because your mezz will resist and potentially break otherwise on the higher level mobs.
the hardest part would be pulling some of them to somewhat of a safe spot because you will likely have respawns trying to kill something like Froggy, It will take a long time unless you have some decent gear, otherwise you just wont much damage before you have to mezz and heal. fungi/singing steel bp (both) are great items to have. Everything in karnors castle you can just pull/train to the zone in (as long as nobodys there
[You must be logged in to view images. Log in or Register.]) and mezz/snare, zone out, and retag.
a fully decked out bard on p99 might be able to solo some tougher non mezzable, summoning mobs by kiting and sustaining with regen songs / singing steel BP and slowly killing with chants or damage shield. I know on live during PoP era i could do this on the dark elves in kithicor that dropped RBG, twisting regen and singing steel bp and innate tankiness i was able to outregen the damage i took from getting summoned while slowly killing them. An unlucky stun might be too punishing here though, but im sure its possible with raid gear, dain ring, vp lute, maybe a few wort pots etc. basically you're a bootleg torpor shaman that runs in circles.
tldr; if you can mezz it you can kill it!