If you want a gambit suggestion, check out mine:
My Current GambitsBack when I took down Hell Wyrm, I was quite a bit further back in the game, so I was lacking a few spells used in this setup (such as Hastega), but for the most part, I'm pretty sure the gambits were just about the same. It didn't really make things much worse for me. Actually, I thought Hell Wyrm was rather easy, but simply a long fight. (A good hour or two, I think.) Honestly, with the point of the game you seem to be at, I doubt you'll have much of a problem with him, as long as you've got up-to-date equipment and a decent gambit setup.
My strategy was basically just having Basch tank like a madman. Wielding a two-handed sword (I forget if I had the Tournesol at the time or not) and heavy armor plus a Bubble Belt, I just kept him Protected and Decoyed, and occasionally threw Berserk at him. (Probably Bravery as well.) He didn't really take too much damage until later in the fight. Vaan was my secondary attacker, though I forget what weapon he was using at the time. Ashe was basically just keeping the party healed, and attacking if healing wasn't necessary. My whole party was only around 60-70 or so, I think.
The main thing that makes Hell Wyrm a bit easy is the fact that whenever you want to, you can just walk out of the fight and heal and whatnot, then simply walk back in and continue where you left off. (I think there's actually a counter on the screen that keeps track of how many times you leave and re-enter. Not sure why. Probably just so you can keep a record.) You probably won't need to do that until about halfway through his health, though, and depending on how your characters are, you might not need to at all.
That's all I can really think to suggest... It's really just a kinda lengthy fight, but not too difficult until toward the end.
...Oh, yeah, I think he's got some Holy-elemental attack that's pretty powerful, so having Holy-absorbing equipment would be nice. A Sage's Ring on your healer works wonders, as it halves MP cost as well as absorbs Holy, so they'll not only survive the attack, but they'll be healed by it, and be left to heal the others who are damaged by it.
Other than that, not much comes to mind. Good luck, though.