Iron Brigade

A bronze medal is awarded for completing a mission, so you will get this when you have finished all 15 missions regardless.

Basically this is a freebie for finishing the game.

Finish all 15 levels and you get a bronze medal just by surviving and making sure whatever structure your defending also survives. Check the gold guide for strategies on levels you struggle with or get a friend to help you as having friends join in (to my knowledge) does not seem to increase the number of enemies.

Important!!: This is a buggy achievement. Once you beat the last level continue through the cut scenes and just stay on you ship. The achievement finally unlocked for me about 5 minutes after I had earned it...

There really isn't much to say. If you finish a level you earn a bronze. So unless you die you will earn this by finishing the game.