I agree on the annoying IA part with Firion, OK, Bartz, Terra, Kuja and Golbez (well, CoD too, if having a block or two). These are the most annoying to fight against for me.
Personally, I prefer to fight some other character than Ex-Death for levelling, just because of all these items you need to sythetize/steal. But that's just me.
I don't actually mind fighting Firion, OK, Kuja or Golbez, even when they're spamming HP attacks. The only one out of them that actually has a fairly hard-to-avoid HP attack right now is Golbez with Genesis Rock. Firion's Straightarrow is just laughable (though I've heard Shield Bash is nasty), neither of Kuja's current HP attacks (Flare Star and Seraphic Star) are that hard to avoid, and Onion Knight often falls short with Blade Torrent and misses with Comet, though Firaga is a bit more dangerous.
They all have their annoying moments (what character doesn't?) but for some reason they just don't annoy me as much as Bartz or Terra. I just don't know why that is. Terra has an annoying voice actor and quotes, but I don't know what it is with Bartz...
As for levelling up, I fight against all the different characters I can because I think it gives me better experience than just fighting Exdeath all the time, since he's just an easy target in the hands of the AI. I don't even do the Exdeath levelling-up trick, because I want to get properly used to each characters' moves and master them while levelling up. I'm not even bothering with getting certain items yet...
