Thor by himself was able to channel the storm's fury in a blast capable of destroying a single secondary adamantium Ultron. Storm isn't Thor (obviously), but her and the rest of her teammates can come close to, if not meet that damage output by Thor.
True, but the Avengers engaging that Ultron were Cap, Iron Man, Black Panther, Thor, and Firestar. X-Men have more to work with, and arguebly, more versatility at their disposal.
There have also been primary adamantium versions of ultron... An the only way they were able to defeat that ultron was by getting their hands on vibranium.
Secondary is not as durable as primary...
And kitty has problems phasing through dense materials... Such as adamantium... However as a suicide mission she could phase into ultron which would disable that gizmo inside ultron that allows him to move etc..
Other than that one an only option... Ultron wins IMO....
__________________ Quotes from Hia8:
"I claimed that the science is sometimes faulty."
"You don't understand. This is fiction. That means none of this stuff really happened."
"There is no writer to purposely ignore a character's natural ability just because it suits the story."
"in some cases because the writer knows that Character A will dominate Character B easily and refuses to allow this to happen for the sake of the story."
Ultron could probably regenerate from being Kitty invading his body, Scarlett Witch short-circuiting all his circuits didn't finish him off, its only thanks to the matter arranger that she's able to take him down.