ww barb can solo cruci up to at least lvl 13
bowa can supposedly solo cain as well
im pretty sure a good druid could solo cain
wwsin wrecks anything outside of cruci pretty much and can at least tank well enough to kill cain with 1 other player.
im sure theres other ones, i havent really had the opportunity to play some of the other builds like fire sorc and summoner and stuff.
im not sure if any build can solo the entire crucible. but theres 2man teams that do it supposedly.