The last boss must always be dangerous and conceived to be able to screw you up no matter how much grinding you do.
You should always be able to push the odds to your favor with good strategy but never be able to be sure 100% to succeed.
I think that's a pretty stupid philosophy, to be honest. Having a boss that might be impossible for you to beat if you are unlucky and thus locking you out of an ending to a game you've played for dozens of hours is just a bad thing to do.
Most people don't finish their games anyway, there's no need to punish the ones who chose to continue playing till the very end.