This happened to me as well. In Explorer, best of 3. I had MOM on the field, under my control, and my opponent cast collected company. Found, Youthful Valkyrie, and Righteous Valkyrie, putting them onto the battlefield, both ETB effects went off giving my opponent the life my opponent the life to survive my attack, and the needed power to kill me exactly.
This happened to me as well. In Explorer, best of 3. I had MOM on the field, under my control, and my opponent cast collected company. Found, Youthful Valkyrie, and Righteous Valkyrie, putting them onto the battlefield, both ETB effects went off giving my opponent the life my opponent the life to survive my attack, and the needed power to kill me exactly.