The Fly Guyskillshot can be achieved by leashing, kicking, or sliding an enemy into a group of blue Electroflies present in the area. Timing is key when performing the skillshot as the moving flies are relatively hard to get lined up with the victim. A fly must be hit directly by the enemy for it to work.
It is also possible to do this with the Head Hunter's Charge Shot if you quickly guide the enemy into the swarm before he detonates. This will award both the Fly Guy and Early Retirementskillshots.