Just share to friends/union and hope they kill it for you while you do the most dmg you can, you wont kill wind ragna with off-element easily (let alone solo it), sorry but thats not happening (unless you are on Maru's lvl of whaling and even so im doubtful), relying on debuffs is your first mistake, she has lower base defense but high debuff rst for something, people are getting good results using joan with fire, but since you arent using fire, you are losing on 25% reduced dmg, all in all, fire is the best choice by far, relying on debuffs for dmg and specially atk downs for stalling is bad (you want dmg cuts against this boss), im afraid you are experiencing the downsides of mono element path.