Instructions
Start the battle with Meril by fighting the masked kid in Level 15. She will show herself with a mask and has two spirit hunters next to her, also with masks on.
Place your troops next to the two spirit hunters to defeat them to leave Meril by herself.
Attack her with your fighters until she has dropped down to below half-health.
Take Yoh and approach Meril once you reach Level 16. It may take a few turns for Yoh to be able to reach her but continue to move closer each turn.
Protect Yoh as he approaches Meril by placing Trey and Lee around him to fight off any enemies that may attempt to take him out. Wait for Meril to attack Yoh and attack her back to deal enough damage to make her quit.
Make Yoh speak with Meril for the battle to end. Once you reach Level 19, use Yoh's "persuade" ability on her to make her realize that she has been brainwashed and she will realize what is going on and stop fighting you.