Thank you, Cortez. My own strategy didn't need too much modification. What I lacked was the Gaian Rings. Now that I have them, I can consistantly slay the Adamantoise without summoning Bahamut. I stuck with Lightning, as she's better than Snow at everything except Sentinel, and if I'm not using a Sentinel, no need to bring him in. I did have to give up Vanille's Random Instant Chain ability for this, but not Lightning's, so she's still able to just randomly instant chain the legs while I'm casting my buffs. Huge help.

Only remaining problem is their ability to hit my whole party with Daze, but I guess that's just what I'll keep Bahamut on standby for.