At the start of the fourth level, you will have to work your way over a chasm. On the other side you will find a rune floating in the air with a symbol on it in green. Approach this and press
to activate it and get taken to challenge to defeat 15 imps in a short space of time. Each kill gives you 2 seconds for a gun kill or 4 seconds for a glory kill.
Once 15 are defeated you will unlock the trophy and get an upgrade rune you can use to improve your shotgun