Okay, so let's puzzle this out here.

How you even gonna try and call me second best?
This seems to indicate that the answer is considered the "second best" of its category, either in-universe or out-of-universe.
I got the baddest one-upped by a factor of six.
So in other words, the answer is six times as good as the best of its kind? Or maybe it has six times a specific attribute more than something else... The "baddest" could mean literally "baddest" as in "worst", though, so it could mean that it has six more of something than the worst of its kind. Hmm... Maybe armour with Materia slots?
If I'm on your side, you can bet I'm gonna please ya,
So this is something that would be good to have, essentially. So probably either an item, armour, or accessory. It could also be an enemy, "on your side" referring to either a skill you can learn from it with the Enemy Skill Materia, or taking control of it with the Manipulate Materia.
Blastin' em like a green-haired chick with amnesia.
This line is my stumbling block. It's been confirmed to refer to Terra Branford, but I can't really see a link between her and the rest of the riddle, and we've already established it has nothing to do with magic, so by assumption the answer also has nothing to do with magic, and it's not a weapon, which leaves me wondering what "blastin' em" means... Unless it's an Enemy Skill or an enemy that "blasts" things. It might be referring to when Terra was riding the MagiTek near the beginning of the game... that had guns, as far as I recall. So it might be an enemy with guns, or something...

I don't know. Have I hit on anything here, or are my shots flying way off the mark?