Its been a long time since I fought that Ultron, but I believe Sym Supreme would be viable as he could keep the buffs off Ultron. I know back in the day there was a Loki strategy also, where you get to SP3 and then spam heavy to keep the curse up (which causes Loki to steal all the opponent buffs). The other possibility is to awaken Void and let him reverse the power gain or healing. I suspect Warlock would also work if you can play aggressively enough to keep infections up.