Go to driftveil city and go to the poke-center. Go to the mart in there, and talk to the person closest to the door. Buy about ten net balls from him. Then, go to route 6. There's a 25% chance to run into a karrablast, and a 5% chance to run into a shelmet. When running into one, use a net ball without damaging them at all. It's almost a guaranteed chance of a catch. After getting a good amount of the two, go to the GTS. Deposit a shelmet for a karrablat, or a karrablast for a shelmet. That trade will get an evolved form of the pokemon the trader gave you. The trade should happen in about an hour, in my experience. Make sure to put the trade as either gender, any level to get the trade to go through fastest. After depositing, search for nice shelmet trades for a karrablast, or vise versa. You should get some nice customers at the join avenue, and some fresh Accelgors and Escavaliers you can trade off for even better pokemon.