I know this sounds funny, and like a lie, but I defeated a Arceus-Grass with Clamperl once (Before it got knocked out, though). In Anything Goes.
Moveset depends on your tactic.
The Offensive was the one who took out Arceus. (All by itself)
Item: Deep Sea Tooth
EV's: 252 Sp. Attack, 252 Sp. Defense, 4 Defense
Nature: Modest (+Sp. Attack, -Attack)
Hidden Power Ground
It was also pure luck the Arceus used Ice Beam and then again, and then used Judgement and got knocked out by Clamperl's third Ice Beam.
The Defensive get's an Deep Sea Scale (so only one scale protects you from a Hyper Beam?)
Item: I already mentioned it
EV's 252 HP, 252 Sp. Attack, 4 Sp. Defense
Nature: Bold (+Defense, -Attack)