An MMG is a Mounted Machine Gun and an MGL is a Mounted Grenade Launcher. You will find these throughout the missions and you will recognize them as they cannot be moved from their position and you will likely have killed an enemy already using it, where you would have had to flank them to kill them, due to the amount of armor covering the front of them.
To enter them, press and hold and you will have control of it. Kill 100 enemies using either of these weapons, or a combination of both and the achievement will unlock.
Kills from the helicopter mounted mini-gun, during the mission 'Narco Hell', also count towards this.
*Note. You can find a place with either the MMG or MGL and get as many kills as you can until the scene ends, then reload the checkpoint and repeat the process, until you unlock the achievement.