Buy the "C4" skill under the "Item" skills category. It is also helpful to have the "Diversion Lure" skill because it makes it easier to lure enemies together. A great place to kill seven enemies with a single C4 blast 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. One of them is walking around, and the others are standing still. Throw the C4 when they are all close together, then hold [Throw] to detonate it. It has a large blast radius and should kill them all. To make this a little easier, throw a Diversion Lure first. The enemies will investigate it and get close together. 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, throw the C4 near them, and detonate it.