Start up a solo Horde game on Dead End with your settings on Cadet and 4 players worth of zombies. Now as the game starts run forward to the Preacher weapon, pick it up and start shooting the zombies who are clumped together to increase your combo to about 50, Now run back to the starting position and try and line up a 3 in 1 headshot with your sniper rifle(I used the Kar98) which should give u well over 5000. If you don't get it by the end of wave 1, Die and restart because wave 2 has those annoying helmet enemies. My shot was 8850 with a 58 combo. I struggled getting this achievement until I used this strategy and I'm pretty sure there is no other easy way, Hope this helps anyone finding a good spot to do it!!
EDIT: As Ehrenfried pointed out, Killing the zombies first who spawn in the middle will make more zombies spawn in the light/mist field making this easier!
thatnoseyparker added you can use explosives to add to the combo so use everything at your disposal!