How do you defeat an elder drake.?

Equip a gunner with cat nip accsessory and make sure she has yellow health. Equip her with highroad winds Garment Grid and use trigger happy. Does around ALOT! Of damage as it is 9999 per hit.

Alternatively, use the gun mage ability to deal quadruple damage to drakes on it

All you need is berserkers they have bad aim on berserkers but if they do hit it's a one hit kill have a white mage cast protect have a berserker and a samari just have the berserker attack and have the samari use zantesu and try and one hit kill the elder drake or if you've beaten the boss on floor 40 and you get the catnip equip that to someone go into battle(not with elder drake)and almost kill or kill and cast life not full life on that character and than run and fight the elder drake have the gunner with the catnip use trigger happy lvl 2,or 3 and instantly kill him.

you can aslo use the berserker DS and just evade and counter but this takes a while if you havent been leveling well

If you're lucky enough to get the mascot dress sphere, then level all three girls so they all have auto shell and protect (you have to get shell before you are allowed to get protect). Having auto protect is what really helped me, plus I had two ragnocks equiped so I had two healers.