When i was about 9 i used to have trouble with Rufus because i didn't know i could re-equip Cloud after the fight in the Elevator.

Then the next one i'd have trouble with is Gi Natak (before i realised you could use the x potion) Lost Number and Materia Keeper, then Godo and Demon Wall (I hated leveling up, i found it boring lol)

The rest of the bosses after that never gave me a huge ammount of difficulty