1) Load up the mission 'Breaking Point' in either solo or co-op campaign.
2) Play through the mission until you come across Japanese soldiers with mortars.
3) Clear one of the mortar pits and jump into it.
4) In the pit you will see mortar shells on a crate.
5) Hold "X" to pick up the shell.
6) Press "RT" to throw the shell at Japanese soldiers.
7) Keep throwing the mortar shells at Japanese soldiers until you kill eight soldiers.
8) The achievement will pop up straight away.

Notes: Don't throw the shells too close as they have a very big blast radius.
An unlimited amount of mortar shells can be thrown.

Here is a video tutorial: