If Mirror Mettaton was capable of pity or empathy, Legion's words would strike a tone in his heart. As it happens, he is the latest model with such useless functions removed, so he instead thinks. What would be the best way to destroy this? Hm. He'll need a little more, a vulnerability. Or make one. That could work too.

"I'm the only one of my kind back home. And everyone loves me. I'm the largest sensation to ever happen to Monster and Humankind. It's a... pity your people don't treat you the same back home."

He expresses his "pity" with the smug air of someone who knows they have it better than you, and relishes in it. Make this robot think that he enjoys that they have suffered (which he kind of does, if he were asked to think about it.) and he'll be able to watch this "friendship" wither and die.