This is my own guide that I recently made for this achievement.
First off, you should know that to obtain this achievement, you NEED two controllers.
Be far enough through the story so that you have unlocked both Captain America and Human Torch as playable characters. Now this achievement makes it seem like you have to play Co-op and go into a mission. But it's much easier than that. Once you're at the Helicarrier, make sure both your controllers are logged in and switch one to Captain America and the other to Human Torch.. and the achievement is yours!
Watch the video as I show you exactly how to do this. If you have any questions, please leave a comment and let me know if there is anything I can do to improve my solution, thanks!