are you kiddimg?^^^^^
gaara is probably one of the most powerful ninjas in the show.....he could own some of pains bodies with those sand techniques like he did to rip deidaras arm off.
True, Garaa IS really powerful, but remember that Diadara was probably one of the weakest, if not THE weakest member in Akatsuki.
Pein is miles ahead of Diadara in every category, and yes, with help, he could kill a few of his bodies but you could say that about anyone.
Naruto, with help, could kill some of Pein's bodies, Kakashi, with help, could kill a few of Pein's bodies.
You gotta remember, Pein can't be sneaked up on, as he can see everywhere, so his sand has already lost a lot of it's advantage because being able to attack from all angles doesn't mean shit when fighting Pein.
Also, we don't know if Pein can use Genjutsu, but assuming he can [Since his eyes allow him to use all jutsu] Garra would have no defense against it, thus almost instantly ending the fight.
Don't get me wrong, I respect Garaa, I think he's one of the most powerful ninja in Naruto considering how young he is and how much power he already has, but Pein has just to much at his disposal to be taken down by JUST sand attacks.
Oh, one other thing, now that he's lost the Shukaku, Garaa has to control the sand with just HIS chakra alone, which severely limits the amount of attacks he can do and how long he can do them for.
To be honest, I think Garaa doesn't have the power to control the sand anymore, he relied to heavily on the sand and the Shukaku and didn't train his body at all.