Rocket Riot

For this you need to complete a level that requires you to destroy objects without killing a single enemy. The easiest level to do this on is "Level 3" where you have to destroy the boots. Be patient and aim carefully while destroying the boots. If you accidentally kill an enemy you can always retry in quick play.

A destroy the object level is one in which you have to destroy different types of objects to complete the level. Just use your rockets to demolish all the objects without hitting an enemy. Enemy friendly fire that results in other enemies dying is okay. This is ridiculously easy to do on Level 3 of the main campaign. NOTE: Level 3 has a few specific enemies that you will miss if you complete it without killing any enemies, which are required for the "The Things I've Seen" achievement. Make sure to replay it an kill some of the enemy types to unlock them.