Analysing Bulbasaur, Pokemon X/Y[]
Bulbasaur gets two abilities, Overgrow, which powers up Bulbasaurs grass type moves by 50% when it falls below a third of it's max HP and Chlorophyll, which doubles it's speed in the sun.
It get's a nice variety of moves and my favorite strategy practically is useless if the opponent resists grass type attacks.
Ability: Clorophyll (It's a Sunny Set)
Moves: Sunny Day, Amnesia, Synthesis, Solarbeam
IV's: Flawless without Attack
EV's: 252 in SP Attack and SP Defense with 4 in Speed
Nature: Modest
Item: Heat Rock
First set up Sunlight with Sunny Day then you should get to a decent speed from Chlorophyll. Then you can set up a few Amnesia's to boost your SP Defense. By then you should note that you have to keep the weather Sunny so keep using Sunny Day ones the sun goes away (Heat Rock keeps sun lasting longer). By then you can use Synthesis to heal the lost HP healing more when it's Sunny. Then Solarbeam with high SP Attack and it doesen't take a turn to charge because of sun works nicely.