This achievement is not as daunting as it may first appear to be but it requires a little bit of dedication and fair share of luck.
First and foremost: Only the following defensive operators can use C4:
Pulse (FBI SWAT)
What you want to do is leave the room where your objective is located in and head towards one of the attacker spawns that is relatively far from your defensive objective.
In social/casual game modes the game will most likely prevent attackers from spawning very close to your objective. (contrary to ranked where attackers can pick their own deployment location).
Now plant your C4 on one of the barricaded windows or doors that are most commonly used by enemies. A lot of enemies do not expect defenders to be that far away from their objectives and as long as one of the camera carts has not spotted you there is a very serious chance that the attackers will just carelessly bash in a door or window.
Once you see a hole appear in the door that you have rigged with C4, detonate it for at least one easy kill ( i've regularly gotten double and triple kills this way).
This requires a fair share of luck. If nobody has entered your window after 10-15 seconds, pick up the C4 again and start roaming the area. You are now in a spot where none of the attackers will expect you so you can attack them from behind.
You can now attempt to do 2 things:
1) If you find yourself behind an enemy you can either toss your c4 on him and just blow him up. This poses the risk that your enemy moves away from your C4 or suddenly turns around etc.
2) Shoot him down to an injured position and then chuck a C4 on him while he is downed. This counts but comes with the risk that his teammates are now alerted or your teammates finish of the downed enemy before you could.
If this is your playstyle you should get this achievement relatively easily.
- Good luck