Hmmm...I never managed to hit it back at him that much. I always got bored waiting for him to toss another blitzball after being hit...
I recently started a new file, in which I'm going to try and do the 'get to level 100 on the first day on Destiny Islands' challenge, and I've found that at the beginning, Selphie gives out a lot of good EXP, too. If you wait for her to do that attack where she sort of jumps at you, and you block it, you get two tech points. Then, if you run around her to the other side of the beach, she does it again, and you get another two tech points. If you keep doing that, then you can gain levels in no time.