Load the mission 'Oasis' in the second act 'Africa'. This is the mission where the Knob variant of Monovisions is introduced. They are very weak and generally die in a single bullet. They travel in packs of up to fifteen at a time so killing them with an explosive is very easy.
Alternatively, you can equip an Artillery weapon and walk close to a Monovision Eruptor and wait until a cluster of enemies spawn. Try to aim in the middle of the group and you should unlock this easily.
This can be done with any form of explosion created by the player.