This is indeed the first time you face off against Devola and Popola after answerings the dove's riddles.
The fight is pretty straightforward. They teleport around constantly so you want a weapon that is quick. I recommend the spear (phoenix spear is a very good choice) as it offers the highest mobility due to its special attack. As soon as you see them sinking into the ground, spin around to look for where they are emerging, close the distance, and lunge with the spear and then get in as many stabs as you can until they sink back down. Magic isn't necessary at all if you have learned how to fight with the spear effectively. They teleport around so much and if you don't hit them soon after they emerge they will start shooting projectiles at you which you have to deal with in one way or another if you are trying to charge up some sort of magic attack in your strategy.
Spears seem quite awkward at first, but I use them probably 85% of the time because they just allow so much more mobility than the other weapon types and leave you open for attacks very infrequently. You can't even be damaged while you are in mid-lunge. I think I got this on my way to ending C, although it's probably perfectly doable on your Ending B run. As always, you'll want to skip cutscenes with start if you are gunning for a boss time trial.