This is basically Power vs Speed! Mothra is far faster than Legion to the point where Legion really only gets one shot against Mothra. Of course she has a barrier which helps. Mothra, however has had many power ups over the years. Light Speed, Armor, Rainbow, Leo. With all of these at her disposal Mothra is definitely a force to be reckoned with. She also has some pretty tough defense and in the end takes the win. Mothra wins.