I didn't skip any battles at all... until Chapter 11. Seriously, I hate that giant blue sphinx-like enemy that summons a sword that does enormous damage. I beat two of those guys, then I started skipping 'em. And, of course, I never bothered to kill all monsters in the Archylte Steppe (especially the Adaman...-monsters).

I also skipped the Juggernauts (giant robots with flame-throwers, insane HP) when I first reached the Mah'habara underground, but when I backtracked because I noticed I skipped the whole sidequest part, I decided to kill the one that blocks the path to a Cie'th Stone. Ironically, that Cie'th Stone wants you to kill yet another Juggernaut.

[completely offtopic] @finaloblivion Isn't the last line of your signature outdated? [/completely offtopic]