There are multiple ways to get this one. You can either launch explosive canisters at enemies using Kinesis, or you can use an explosive weapon.
Your best bet for an explosive weapon would be on the Pulse Rifle blueprint. You get this blueprint in Chapter 9, right after the semi-boss fight. After you go up the ramp you will enter a small shed type room. In the second room, there will be a ladder. Go up the ladder, and the schematic will be on the desk to the left. It is very cheap to make (125 Tungsten, 450 Semiconductor, and 850 Scrap Metal), and it has a grenade launcher for the bottom tool. Just slap that baby together and go kill 30 enemies. Happy slaying.