Almost completely unmissable this.
You will encounter your first rune trial near the beginning of Mission 4 "Beginning of the End". You will climb up a series of platforms, once near the top you can jump across and head through a tunnel with a rune/portal appearing directly in front of you as you bear right. Head over to it and activate it with cn_RS.
You will be taken to a rune trial where you have 10 seconds to kill 15 imps with a shotgun. Don't worry this is easier than it sounds, since you gain 2 seconds for each imp kill with the shotgun, and 4 seconds for each glory kill. I managed this with 8 seconds to spare without even trying. Complete the trial and the achievement is yours.