you don't beat the arceus you catch it.

Well it's hard to tell you ~.~
Arceus can change to 17 type because of its ability : Multitype (Change type depend on Plate held)
Did you know plate that you collect on D/P
It can change Arceus's type. For example:
Fist Plate : Arceus Fighting.
Earth Plate: Ground Arceus
Dread Plate: Dark Arceus
Check this site for more information http://bulbapedia.bulbagarden.net/wiki/Plate
If Arceus is not holding anything or different item, its type is Normal. And easy, normal's weakness is fighting

.If you don't know what type weakness. Check this chart
http://img372.imageshack.us/img372/3961/dafwv6.png
The Black pokeball is 1/2 x to that type
The Red POkeball is 2x to that type
X mean do 0x on type