Buy the "Mine" skill under the "Item" skills category. It is also helpful to have the "Diversion Lure" and "Flashbang" skills. A great place to kill seven enemies with a single mine is in "The Deserter" (Montuyoc province) main mission. There are seven enemies standing close together at the first objective of this mission. Mark them with the drone to see where they are. Throw a Diversion Lure to get them closer together. The lure only attracts enemies within a certain radius. Make sure you throw it far enough so enemies near the water can hear it. The enemies will come to investigate and stand around the lure. Throw 1-2 flashbangs to blind them, run in, and quickly place the mine. Then, detonate the mine to kill all of them and get the "A Better Mousetrap" achievement. Compared to other explosives the mine has a rather small explosion radius. If you still have trouble, buy the "Explosion Radius" epic skill from the "Item" skills category. You must buy the first upgrade of all item skills to get it. This increases the blast radius so you can hit more enemies at once. If something goes wrong, kill the guy with the yellow quest marker or let him escape to respawn you near the enemy group. This same strategy can be used in other places that have seven or more enemies. Just throw lures until they are all close together, stun them with flashbangs, place a mine near them, and detonate it.