This might not be what you're looking for since Shaymin is banned in some places, and not at all easy to get to if you didn't have access to the wifi event a while back, but it's the best grass-type attacker I know of. Bulky, Fast and Powerful all balanced into one package.
Shaymin @ Choice Scarf
Modest Nature
Natural Cure
-Seed Flare
-Air Slash [Tutor in Platinum while Sky Forme]
-Earth Power
EVs: 136 HP, 252 Sp. Atk, 116 Speed
The choice scarf requires some prediction skills, but there aren't many pokemon safe from seed flare. The better side of this set is that several pokemon think they're safe [Blissey for instance] when they really aren't.
EVs allow for you to outspeed pokemon up to base 130 speed as well as beat Heatran to the punch even if it has a choice scarf. You can sacrifice HP EVs for more speed if you feel the need.
Seed Flare is the primary focus here, sporting 180 power after STAB and a 40% chance to not just reduce your opponent's special defense, but halve it. Special walls without an outright resistance to grass won't be alive for very long.
Air Slash will cover fellow grass types and also combos with choice scarf well due to the flinch chance.
Earth Power will threaten fire and steel types that may switch in.
If you're willing/capable of getting a good hidden power on Shaymin, then it'll probably be the best for the last slot. If not, then the next best choices are Rest, Grasswhistle and Sleep Talk. You won't find many chances to use any of these, but it's better to have even a slim chance at getting use out of a move than taking only three attacks into battle ;P
Rest might help Shaymin stay alive longer, and combos well with natural cure. Send Shaymin in on either a helpless opponent or anything else you know will switch away and use rest. Switch Shaymin out and the sleep will be cured by its ability.
Sleep Talk allows Shaymin to absorb sleep even better than just with natural cure alone. You'll be forced into a switch after you wake up, but otherwise a decent choice.
Grasswhistle high risk and high return. 55% accuracy isn't worth using in my opinion, but Shaymin does have enough bulk to live through basic attacks even if it misses the first go.