I kept reloading that round of the Showdown where one of the last few matches pitch Jezabel against Merryll.
I wished Jezabel VS Merryll is the first match of that round so I can keep reloading faster without going through other fights.
So, I thought, to unlock this Steam Achievement, I have to inflict "TAMED" on Merryll.
God knows how many times I tried!
Actually, and I hope I'm the only dumb one around.
You just need to defeat Merryll with Jezabel.
Be it Tricking him off the Arena, or KO him with Skills DMG.
Stupid me~