Leafeon - Swords Dance, Energy Ball / Leaf Blade, Hidden Power Fire / Synthesis, Return
Roserade - Sludge Bomb, Leaf Storm / Leaf Blade / Energy Ball, Hidden Power Ground / Ice, Sleep Powder
Victreebell - Sludge Bomb, Energy Ball, Growth, PoisonPowder / Sleep / Stun Spore
Shiftry - Energy Ball, Dark Pulse, Nasty Plot, Hidden Power Ice
Abomasnow - Ice Punch / Blizzard / Avalanche, Earthquake, Seed Bomb / Wood Hammer
Leafeon, Roserade, and Abomasnow can be obtained in Sinnoh I think. Abomasnow is more versatile, but has 7 weaknesses. Roserade is relatively weak in defense and HP, while Leafeon is also weak in HP, but weak in Special Def. I hope you pick well, and experiment with them.

You can obtain another Energy Ball at the Battle Frontier which costs 64 BP and requires you to beat the Elite Four, and also on Route 226.

Roserade is youre best choise

The best one that I think is TORTERRA he is a massive grass pokemon he can learn energy ball, wood hammer, Leaf Storm, and Frenzy plant if you go threw the battle island.

Oh yah and torterra can learn Giga Drain and Earthquake and Crunch

If I remember right, you can buy an Energy Ball TM with either Battle Points in the Frontier, or from the Game Corner with Coins.

As for a Grass type, try this Breloom:

Breloom @ Toxic Orb
Poison Heal

Spore
Substitute
Focus Punch
Seed Bomb/Leech Seed

Also, do you know about EVs? They can help you a lot in battle.

TORTERA!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
Leaf Storm
Crunch
Earthquake
Frenzy Plant

Roserade