You really need a balance. RPG's can be let down if they are too easy or frustratingly hard. I think all of the FF's have a good balance, it just depends on how much effort the gamer is willing to put in.

With FF7 I found it challenging, and the first time I fought Sephiroth I got kicked. However this was my first FF and I hadn't done a lot to prepare for the final battle. After getting some more HP pluses and using a bit more strategy I defeated him but it wasn't a pushover. The next time I played it I did all the chocobo breeding and got Knight's of the Round and he wasn't anywhere near a challenge.

With FF8, a bit past halfway through the game I couldn't defeat one of the bosses. My brother told me I needed to do more junctioning and afterwards I didn't have too many problems. Ultimicea was easy with the spoils I reeped from Ultima weapon.

The other FF's I've played pretty much followed the same thing, whereby the more time I put into doing extra stuff, the easier the game became. I think this is probably the best way to do it, as the difficulty of the game is more or less determined by how the gamer plays. That's the only problem I can see with setting specific levels for bosses, the game wont change to accomodate for if the gamer has leveled up a lot, or simply rushed through. If Sephiroth became harder the moment you got Knights of the Round, and Ultimicea became harder once you defeated Ultima weapon (not just with increasing level) than both games would have been a lot more challenging.