Nocturne can.

I recently saw a level 16 Xin Zhao do it quite nicely, with about 50% HP left.

any ad carry, if they are fed that is.

i've done it with tryn, full crit glass cannon build ie, 2 pd a blood thirster, boots

ive solo'd baron with orianna

you can have any ad carry do it if they are lv 18 have infinity edge phantom dancer berserk boots 2 bloodthirsters fully stacked and what ever the last item is doesnt matter... just make sure you dodge the bubbles that knock you up into the air and you maybe have to use ignite but i seriously doubt it... solo baron isnt a big deal when me and my friends play we do it all the time... one guy solo's baron while we have it warded well enough that he can escape if the enemy team decides to come and take it.

Seen it with a fiddlesticks, but that was a long time ago.

Fiddlesticks. Especially with blue buff.

Fiddlesticks with blue buff should have drain up all the time as long as ur draining for its full duration.

I've seen a Heimer almost do it using just turrets. The problem was after about losing 25% of its health Baron would disengage from attacking turrets and rapidly regain health.

Udyr is very easy.