Well my first play through I leveled till 100 and all the bosses are still really easy to kill. I don't see how people would need to leave their levels at like 7-10 just to get weak bosses in order to progress. The time you waste carding every single battle, or running isn't much less than just spending 1-3 hours leveling on the island closest to hell. You can get from 30 to 100 in that time. Plus Ultimecia is pretty weak if you asked me. Just let squall limit break her a couple times, limit break griever, *use holy wars if necessary*, more limit break ' ooo a lionheart gg '. It's no real feat defeating Omega at level 7. It's not even REALLY hard at level 100. All you need is Aura/Holy Wars and Omega is pretty much doomed.
