actually, you must be fighting the Time Devourer on top of Terra Tower. Well, there are two Time Devourers. You're fighting the one in Another World, where its form is the dragons fused together. the REAL final boss is the one at Opassa Beach in the home world, and its form is Lavos with Schala trapped inside.

and so you don't have to worry about what Edgar said about casting those elements in an order for now. against the other time devourer you might have to

but anyway, i just used Serge, Kid, and Glenn (or Karsh, if i didn't get Glenn). I equipped everyone with Dendorite or Rainbow items, lots of healing elements, i had ONLY two revives, and strong level 4-6 elements. (iceblast, magmaburst, aeroblaster, meteorshower, deluge, inferno, carnivore, holylight, freefall, earthquake) I hit her with the opposite element of what she is at the time, and she switched between colors pretty quickly.

And if you're still having trouble because of her elements doing tons of damage, just cast Diminish at the beginning of the fight and you'll be fine. (Diminish halves all element damage [it says it does it temporarily, but i find it to last throughout the whole battle])

good luck