Sephiroth is somewhat easily defeatable if you know his attack pattern. If you don't know when he is about to use Sin Harvest, then say goodbye. But my trick to beating him is in the second wave of attacks, when he is running around like crazy and teleporting everywhere. I just focus on keeping the target lock on him at all times and I only hit him when he is about to use Sin Harvest. Superglide is a lifesaver in this situation. All of his other moves are powerful but fall short of the power of the Sin Harvest attack. If you can avoid that you can defeat him with moderate ease. It took me about eight tries to realize what to do, and after that I can beat him every time, no problem.