Being stubborn is a sin. When your first WOW against Arceus missed and you saw it vanish instantly, you should have immediately switched to your own Arceus. You risked losing the match there and then.
Also, HP Fire is generally a better coverage option than Psyshock on Xerneas. Also, if a scarfed Xerneas (scarf gives a +1 to speed) can outspeed a +2 Xerneas, there must be something very wrong with your EVs...