SPOILERS UP TO AND INCLUDING EPISODE 5
The one against Theocoles or however you spell it. I don't really get why Crixus started trying to fight on his own after the guy got up again, but damn if my heart wasn't pumping during the fight.
Also, glorious rain on slow-motion boobies.
END SPOILERS
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XS2 is the only good Xenosaga game - Lady Ashe