This solution is been given FULL CREDIT to SNITTY 1 for it as he discovered it. I tried the other solution 6 or 7 times with the landmine but no go. SNITTY did it with the SNIPER RIFLE (BARRET 50 CAL) which did NOT work for me but when I tried the ROCKET LAUNCHER it worked. (The Launcher was a level 5 yellow but think the others would work as well so try).
At the bottom of the map is a bridge linking the NORTH and SOUTH small land masses, it is a LONG BRIDGE hard to miss. At the south end of the bridge is a large wrecked car accident, several burned and destroyed cars and one large cargo truck not destroyed. Make your way to south end of the bridge and if you look down the bridge (scope or binoculars) you will see a single hunter near a white car (sometimes he is not here so just go to a different safehouse and come back and he should reappear. If you shoot the car in the grill/engine in one shot the explosion should kill the hunter (thus one shot one kill). This hunter respawns here so if you mess up just repeat the possess. Now when you do repeat make sure you do a COMPLETE game leave, if you do not since you destroyed the hunter he will be in his extremely weak state which will not count for the one shot you need him at full health. Leaving the game resets him if you just do a leave game to the game menu and jump back in he will not reset must leave the game completely.
EDIT They changed the area in the last recent patch the car now faces away from you when you look down the bridge but they we're nice and the truck is OPEN and inside is a large gas cylinder which if you hit will explode basically doing what the car grill did so this still works you just shot something different instead.
Hope this helps and thanks again SNITTY 1 for this.
Here is another from PhilipHerpantie, he gets FULL CREDIT for this one. he used a level 5 YELLOW Rocket Launcher on a ORANGE HUNTER (these appear to be the weakest of the bunch). there are several ORANGE hunters in the first area you start, near that bridge in the above solution ( head west of bridge ), I would suspect you would probably want a BACK shot given that is where his battery is but experiment and see how it works if the above does not work for you.
Another method you could try if the above 2 do not work for you is the red propane barrel trick. S4W gets full credit for this, he just planted a red propane canister down and shot it when a hunter got close. I guess you could add a radio or gheto blaster to this and attract one over that way as well, think that would work.