Hi, in Crisis Core: Final Fantasy VII episode Joe and Kaleb say that levelling is completely random. I think I heard that one of them got like 3 level ups in few minutes. I've never had that kind of situation and I smell BS. Guys ff wikia says:
Though it appears that leveling up is random, it is not entirely so. Zack does gain EXP for defeating enemies, but it is a hidden value. The DMW will not line up three 7s until Zack earns enough EXP for it to do so. Once Zack has enough EXP, the DMW has a chance to line up three 7s, but it is not a guarantee. For this reason, the player cannot simply level up by letting the reels spin over and over, they are still required to defeat enemies to earn EXP.
Another thing is about SP points. They say those are useless and I this 100% incorrect. SP points are used for materia fusion. Incidentally episode never touched on materia fusion.