Xander has a bunch of decks in his bag. They each pick one that suits them.
Giles: white weenie
Xander: the one-turn-kill if the draw is right deck, lotus/mox/channel/fireball/lightning bolt.
They play. Giles loses badly his first few games, then cleans Xander's clock, then Buffy's, then gets into a titanic struggle with Willow. Something happens. Perhaps they accidentally defeat the demon by triggering magic with the cards. Or perhaps it's just a silly character piece. Dunno yet.
Two years later, Xander sells his full set of moxes to fund his cross-country trip.
(And yes, I realize that this story idea says very very bad things about how geeky I was in the mid-90s. And no making comments about the past tense is the wrong tense there.)