Unless Doom or Apoc come up with an electronic way to take down Ultron, he and Magneto take it, as being 100% pure Adamantium would mean hes pretty much invulnerable to anything any on the other teams could do. Maybe Ronan rod could absorb all the energy out of Ultron, but considering thats a tactic Thor never tried, Im not sure how likely it is Ronan would even attempt that.