Weezing is an odd one. He's kind of a mixed tank. He takes physical attacks, while launching mostly special moves. He's great for tanking out things, and switching into earthquakes, which in OU climate, are pretty common. He's mainly used for tanking, walling, and disrupting sets.
For tanking, he's got a decent physical movepool, and a great special movepool, ranging from things like thunderbolt, shadow ball, flamethrower, fire blast, dark pulse, Curse, venoshock, and payback- all great coverage moves.
For disrupting, he's got Destiny bond, Haze, Momento, , Pain Split, Will-o-wisp, clear smog,taunt, and torment.
The basic idea of Weezing is to ruin your opponent's sets and make them a helpless victim against you. Just be careful of things like Alakazam that can make short work of him. He works well with dark type pokemon that can switch in on psychic moves for immunity; In the same idea, he can switch in on the fighting types they're weak to.