No prep time. They fight to the death(or until one Pokemon faints). Both are bloodlusted.
@st0nnec0ld: But then it could had use the ground plate so he could had be unaffected by it.
Also even if he didn't had the electric plate, Pikachu can't even scratch Arceus, not even a Volt Tackle.... unless is Red's Pika than it's another story.
@st0nnec0ld: But like I said, Pikachu can't scratch arceus...
Well Arceus is the "god" of Pokemon except for this guys...
Mewtwo, Deoxys, the legendary beats of Johto (since Ho-oh is their god), Ho-oh (IMO Arceus did not created Ho-oh), Castform, Porygon, Porygon2, and Porygon Z because Arceus didn't create them (except Ho-oh but IMO no) but that's off topic.
Arceus one shots Mew.