Real easy way to beat him - use the sword that the old man staring at the rocket gives you in Rocket Town.

I remember having a hard time with raaps at that point in the game, remebered i had a new sword, equipped it, and he went down easy. I'm not sure if it has a hidden element that he's weak to, but it works. Leader of Mortals said it - the Yoshiyuki, i think. it's much higher damage for very little materia slots. Come to think of it, you should equip any weapons/armlets that have few materia slots, since you're not using it, they usually make up for it in another way. It's a start.