They are both equally matched, it all comes down to skill (tell me if you want a story of a duel between them)

There should be a duel. Xander wins, and Mirai leaves him alone forever. Mirai wins, and Xander has to marry her.

That was exactly what I was thinking

Well I imagine they're about equals in sword fighting since Mirai was able to defeat the collector, even if she killed an entire timeline in the process

However xander has a shield, meaning he can block attacks more easily

Xander doesn't really use his shield anymore... he mainly keeps it as decoration... he also doesn't want it to break either

Xander’s probably really backed with support from us so…

The duel is a 1 v 1 between Mirai and Xander

Xander should win because plot

Xander since he has experience with beings stronger than he is

Either it's a draw and they both give up, or they both die. I imagine them as exact equals.