Why does this guy even exist?!? We have Cacturne! And this guy isn't even a Cacturne, it is worse. I like Cacnea better, at least Cacnea can do some damage in Little Cup. With the standard poor movepool of most Grass types, the only redeaming feature is Water Absorb, which makes it a decent switch in on Water types. However, Water types also have Ice Beam. So not that much of a redeaming feature.
Ability: Water Absorb
EVs: Sp Attack, HP, Defense Nature: Modest
Grass Knot: Since this guy is going to switch in on Water types, why not kill those Bulky Waters?
Hidden Power Rock: To deal with those weaknesses.
Spikes: Once they realize that they can't do much to you (if for some reason they don't have Ice Beam) then switching is likely.
Substitute: Water Absorb will heal you.