Use ice spells to increase your mp while dealing weaker damage. Use fire to deal good damage at the cost of your mp. "Transpose" will flip your state from "astral fire" to "umbral ice", or umbral ice to astral fire. In umbral ice your mp will regen. In astral fire it will not regen but instead costs even more mp and you do more damage.

Use fire spells until your mp is almost empty. Then you transpose and start casting ice based spells until your mp is full again. use transpose once more after that and restart from the beginning.

https://ffxiv.consolegameswiki.com/wiki/Astral_Fire
https://ffxiv.consolegameswiki.com/wiki/Umbral_Ice