You only have to play the last 5 waves of each map for the achievement to unlock. Find a good place to hide and slowly take everyone out. Make good use of the remote mines, and the new and VERY helpful proximity mines. Max out your gadget upgrades one your armor all the way and proximity mine the main flow points in each area. For pistols, use the FIVE-SEVEN and I like the AK-47 as my backup. The noise gets them away from the EMP if they are focused on it.
This works best in co-op and be sure to use the double mark and execute. If both of you have 4 people marked, only one needs to start the execute then the other can join them getting 8 kills and most the time you will not get spotted if you are hidden. After that, mark another 8 and have the other player start the execute. This, combined with the mines, will take the enemy forces down very fast!
The only really tough map is St. Petersburg 1. Have both players hang from the inside of the top balcony (one on each side) and shoot out the lights. Mark and execute if your friend right here and the new proximity mines can take out almost 5 enemies at a time!
Use your sonar goggles to mark enemies before the get in the open so you are ready to take them out as soon as they are within a clear shot. If you need to refill your mark and execute, wait for some guys to run past you on the balcony and press B to grab the last one so the others don't see you.
If the EMP takes too much damage on any map or you need your mines refilled, just die. You will reload the wave you are on and the EMP goes back to 100 health and your ammo is completely full! You will still get the achievement when you complete wave 20.
These tips work great on Realistic too, and both achievements will pop if you only do Realistic.