This battle will only happen if you choose to ignore searching for the answer to the question that Tastiella gives you shortly after awakening Shurelia, and instead try to force your way through her in order to fight Mir, which will automatically put you in the path for the Bad Ending.
Tastiella is quite an strong and speedy foe, which is only compounded by the fact that she has innate regen, and her skillset: Gate In deals large damage to a single target and makes her retreat to another dimension, making her completely immune to all attacks; Gate Out deals large damage to a single target and makes her reappear in the battlefield, allowing you the chance to attack again; Holy Light restores a large part of her HP; Force Shock causes large physical damage and delays the turn of the target; and finally, she has a Lv. 3 Stored Attack called Abysmal Gate, which causes extremely large magical damage.
As you might have understood by now, attacking her after she has used Gate In is completely useless, as no attacks will reach her while she is in the other dimension, so you should use that time instead to heal yourself and begin preparing an strong Song Magic for when she reappears. Also, take in account that her regen is still effect while she is the other dimension, and might also use Holy Light to undo all the damage you might have done to her, so the only purpose you should give to your vanguards in this battle is refilling the MP of your Reyvateil until Tastiella uses Gate Out, and also rising the Ambience Field before unloading the strongest song you have at your disposal. Also, make sure to equip the best equipments and Grathnodes you have beforehand to resist her attacks.