YAHF: Way over yonder.
For this particular story, Xander never dressed as a soldier for Halloween. Instead, he dressed as the Superboy Prime featured in the COI Superman tie-in which introduced the character. But, due to the meddlings of Mxyzptlk and the Roman god Janus, the personality which took over and whose powers, experiences, knowledge, and memories were left over once the spell was interrupted were those of the Kal-El of Earth-Prime ca. the end of the Blackest Night story arc. This, coupled with Xander's own character and personality, as well as his experiences before and after meeting Buffy Summers and her Watcher Giles, results in him becoming a formidable individual. An individual which the PTB want nothing to have with, and, after a heavy bargaining session with the 5th-dimensional imp and the Roman deity he's in cahoots with, decide to exile from his own dimension and into an alternate reality of jokesters' choosing. Namely, that of the mainstream Marvel Universe Earth (Earth-616) at the onset of the Civill War arc.
In this story, Kryptonians (SBP) will be compliant with my own personal tweak for an ongoing story: