The problem with FFXIII is that if you don't do any grinding or levelling up when you hit Gran Pulse, you're in for a tough time. if you go out of your way to do some missions and train up, then it really isn't that bad.

Like others have said, the worst thing about the final boss fight is the instant kill, though that only happens in the second stage (of three) of the fight. The final phase has Doom, but it isn't bad.