I'm in Chapter 9 and the bosses so far have been pretty generic (aside form Anima who didn't get much development anyway because it appeared early in the game). They've been primarily either non-descript military machinery or some non-story related lifeform they stuck at the end of the map.

By chapter five they get a bit of a challenge because it's unlikely you'll have full grasp of the Paradigm system yet, but once you work out a Paradigm system/rotation that works for you, they end up being more or less the same.

Other than being long, they're ultimately not tremendously difficult. I've had a much harder time on a few non-boss fights without getting a Preemptive Strike (Falco Velocycle + 2 x PSICOM Aerial Snipers in Palumporum comes to mind).