I was battling in battle subway when my opponent sent out yamask and simisage.
That person used yamask to will-o-wisp on his simisage and it hit.
The question is -"why did he harm his own Pokemon???"
Since the Battle Subway is a place where Game Freak have designed to get tougher and tougher after a while, they also make Trainers in game play certain strategies to annoy you. There are also other like using Swagger on your own Pokemon to make their attack higher. This is how the game is programmed. Nothing is wrong with your game.