Angel was the first to remove his helmet, but Cain started bolting it down after that. Shatterstar and his magic blades cut it off, Wonder Woman knocked it off, a feat also done by Thor, the Hulk, (twice) and Hollow of Generation X got it off.
Nimrod, Beast, Wolverine, one of the Captain Universes, and my personal favorite, Longshot's luck powers damaged the armor enough that Rogue could yank the helmet off, where Dazzler used her lasers to cut the secondary helmet off and BAM, Psylocke for the win.
And of course, Onslaught.
These mind you, are only the times and people that beat Juggy by removing his helmet. He has been beaten back from his goal, (spiderman twice, The Thing, etc) removed from the area/state/planet, and depowered more times than I can count.
For an unstoppable being, he gets stopped a whole lot.
That might be Cain's... Average, slightly less than average intelligence. He considers his power to be forward momentum: if he stops walking, he's vulnerable.
Along comes Taylor, who picks a goal, as in the first chapter, (catch/kill Sophia.) and becomes unstoppable in pursuit of that goal. That would easily explain how she followed Sophia through the walls.