Waiting

I would have liked to actually fight Wilhelm too, but the way they set him up throughout the series, and especially in Pied Piper (which unfortunately never came to the States), it's made pretty clear that it'd be impossible to genuinely beat him. Hell, he doesn't actually die either at the end; he kind of just chooses to reintegrate himself into the Prime Material Plane since mankind had finally proven itself worthy of being able to reach Lost Jerusalem since they finally defied him (with KOS-MOS destroying the key instead of giving it to him).

Technically, it can be argued that you do get a boss fight with Wilhelm, since Zarathustra is a reservoir of Wilhelm's power, paralleling how Friedrich Nietzsche identified with the ancient religious figure Zarathustra, or at least used Zarathustra as an avatar for himself. So you do fight Wilhelm, it's just that you fight an incredibly weak and manageable version of Wilhelm.

A wonderful picture of Wilhelm, though. There's very little fanart of him out there. Sorry for talking so long about this, it's just that I love Xenosaga (as if you couldn't tell. )