For this, as well as many of the other "get 'x' kills" achievements, you do not need to get the final blow. Just get some damage on a player and then have them get killed.
Keep in mind that you can only have one mine active at a time.
The best thing to do for this is just launching your mine into team fire fights. This will work best on Control maps by launching them into the control point, or on Escort maps by launching them at/near the back end of the payload (when on the defensive side, or the front end of the payload on the attacking side). I earned mine on the defending side of an Escort map. Hope this helps someone else!