You will first meet this boss in Temple of Rain but don't get to fight her until you go back to Pueblucho.
You will be back in the ghost world at this point so the village will look different.
Run past the statue and enter the leftmost house before you get to the mansion.
X'tabay will be in the cellar and she will talk to you and try to seduce you.
Whether you answer Yes or No, I think you would still end up fighting her.
She will teleport around the room.
Each time she appears except at the top center, she would blast out a circular pattern of fireflies. Avoid them and in the second or so after the fireflies fly past you, Rooster Uppercut (
) X'tabay, then before she hits the ground, combo hit (
) her if you can.
If she teleports at the center of the room, she would phase shift the room to have spikes on the floor and a platform above it. Jump onto the platform (preferably when you see her teleport sign so you can avoid the spikes) and continue your attack.
Another teleport to the center of the room would revert the room.
At about 1/2 of her health, X'tabay would pause and taunt you some more.
On continuing the fight, she will change tactics and attack you in 3 different ways.
1. Lean forward and strike at you with green flame.
2. Leap in an arc towards you, encased in green plasma.
3. Summon a lightning rainstorm down upon herself.
Avoid the lightning storm completely. Her tell is she would raise her head up and turn red.
When she lunges at you, jump over her flamed tongue and Frog Slam (
) her from above.
When she leaps, avoid and wait for her to land, then Frog Slam her landing point.
At about 1/3 or 1/4 health, she will talk to you some more.
X'tabay will then split herself in two with one sporting a red shield and one a green shield. Both use the same 3 attacks as in Phase 2.
After defeating those two, she will summon a yellow shield.
Defeat yellow shield X'tabay and you will have won.
Listen to her story about Calaca and get the phase-shift power.
The trophy will pop at around the time you finish talking to her.