Or are you referring to the Wyrm Elder battle at the end of Golmore Jungle before you reach the Paramina Rift?
BOSS: Elder Wyrm - HP: 71692 - LV: 27
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If you thought the Tiamat was hard, the Elder Wyrm is going to have a field day with you. He certainly has no interest in fighting fair. We'll begin with the basics: start the battle by launching a Quickening chain (like inferno or cataclysm). This serves two purposes: one to deal MASSIVE DAMAGE to the boss, and second to eliminate those two treants he starts off with.
Buffing up your party prior to the battle is once again, a good idea. He has a regular assortment of physical attacks which aren't particularly powerful, there are two main attacks you need to watch out for:
The first is a fireball blast which seemingly comes out of nowhere. This is really only dangerous if the party member it hits is currently covered in Oil (which, given the boss' pattern of attacks, will likely be most of the time). You're looking at damage between one and two thousand that can easily wipe a single character at full HP. Be ready with Phoenix Downs.
Speaking of being ready, you'll want at least two characters to "be ready" with the Gambit "Any ally: Esuna" given the boss' most dangerous attack:
Sporefall. Similar to Bad Breath in previous games, this inflicts a variety of negative status effects on every member of your party. Esuna will wipe the slate clean, but you may have to be ready to use an Echo Herb on someone so that they can actually cast it.
Nail the boss with your hardest physical attacks. It has a lot of life, and so this is going to take a lot of work, but if you keep these tips in mind then you'll at least have a fighting chance.
(Boss's status attacks include: Slow,Blind,Sleep,Silence,Confuse,Poison and Oil-which he usually follows up with fireball so make sure you have plenty of handkerchiefs since esuna doesn't cure oil).
But if you are talking about the Garuda Boss before Raithwall's tomb follow strategy from previous poster and to make it easier aquire the Eskir Berries when you complete treasure of the sandsea quest for the moogle in the sandscale bank area of the Nam-Yensa Sandsea near save crystal. The Garuda hates the berries and it weakens him. Ranged and Magic (water and thunder)attacks fair better in this battle. Silence also works against this foe but he occasionally casts sleep so either have esuna or plenty of alarm clocks (item).