you can use nearly every champ against Ultron. He has 3 random buffs, power gain, regeration and third one which don't remember right now (fury or armor up), the buffs are greyed out and will be activated when you block or parry Ultron, so all you need to do is avoid block or parry as long as the power gain buff symbol is shown up in upper right corner, because this is the annoying one. the other 2 buffs can be handled quite easy.
Ghost works great, as you can create openings without parrying and procing his buffs. Throw in her synergies if you want an easier time, should be a straightforward fight as long as you watch out for his evade on your non-crit attacks.