Duke Nukem 3D

Good place to get this is one is on the "Raw Meat" level of "Shrapnel City" (First level of Episode 3). The shrink gun is near the beginning of the level and there are plenty of easy targets throughout. The shrink gun is located behind the "Exotica Tonight" sign with the big stripper with the red bikini. After you enter the first building, passing over the ramps surrounded by white walls, go straight and it will be on your right hand side in the hallway. You can keep going back and grabbing the gun throughout the level for more ammo or if you die alot. It has 10 shots so use them wisely. Sometimes if enemies are standing close to one another you can get 2 with one shot so make em count.

Edit: It has been confirmed that multiplayer does not count towards this achievement. Sorry for any confusion.

T̶h̶e̶ ̶o̶t̶h̶e̶r̶ ̶o̶p̶t̶i̶o̶n̶ ̶y̶o̶u̶ ̶h̶a̶v̶e̶ ̶i̶s̶ ̶i̶f̶ ̶a̶ ̶f̶r̶i̶e̶n̶d̶ ̶h̶a̶s̶ ̶t̶h̶e̶ ̶g̶a̶m̶e̶/̶y̶o̶u̶'̶r̶e̶ ̶b̶o̶o̶s̶t̶i̶n̶g̶ ̶t̶h̶e̶ ̶d̶e̶a̶t̶h̶ ̶m̶a̶t̶c̶h̶ ̶a̶c̶h̶i̶e̶v̶e̶m̶e̶n̶t̶s̶,̶ ̶y̶o̶u̶ ̶c̶a̶n̶ ̶a̶l̶s̶o̶ ̶g̶e̶t̶ ̶t̶h̶e̶ ̶4̶0̶ ̶s̶h̶r̶i̶n̶k̶e̶r̶ ̶k̶i̶l̶l̶s̶ ̶w̶h̶i̶l̶e̶ ̶w̶o̶r̶k̶i̶n̶g̶ ̶o̶n̶ ̶t̶h̶e̶ ̶k̶i̶l̶l̶s̶.̶ ̶O̶f̶ ̶c̶o̶u̶r̶s̶e̶ ̶i̶f̶ ̶y̶o̶u̶ ̶p̶l̶a̶y̶ ̶t̶h̶r̶o̶u̶g̶h̶ ̶t̶h̶e̶ ̶g̶a̶m̶e̶,̶ ̶o̶r̶ ̶u̶s̶e̶ ̶t̶h̶e̶ ̶a̶b̶o̶v̶e̶ ̶s̶o̶l̶u̶t̶i̶o̶n̶,̶ ̶y̶o̶u̶'̶l̶l̶ ̶g̶e̶t̶ ̶i̶t̶ ̶w̶i̶t̶h̶ ̶e̶a̶s̶e̶ ̶e̶i̶t̶h̶e̶r̶ ̶w̶a̶y̶.̶ Confirmed by MechaChrome

As mentioned before, go to raw meat level. Get the shrink gun. Now if you want to be cheap, shrink an enemy or two really quickly then get killed. (I found in the water area with the shark and the 3 alien things worked good outside the area you got the gun if you jump down the ledge to the left and use the pipe bomb to blow up the wall). Re-spawn right before you stepped on the first one in the kill sequence, repeat next few kills and die (i shot some alien thing, then the shark, then 1 of the aliens and would then die). So I did 3 shrinks in about 20 seconds, died. Repeating that you can get to 40 kills in about 10 minutes.

Very simple method to obtaining this achievement, as well as the earliest you can get it.

While making your way through the game, during Episode 2 - Mission 1 (Spaceport), directly before the red keycard lock opening the way to the level’s exit is a brighter section of flooring. “Use” the floor while standing on it to be lowered into a secret area, around the corridor to the left will be a Shrinker. Grab it and reach the level’s exit.

Upon reaching the subsequent level (Incubator), proceed forward through the two doors to reach a large darkened room. Immediately save the game (a door will open directly in front of you revealing the enemy you’ll be using for the achievement). Now just shrink and stomp, reloading your save after each kill and you’ll have the achievement in less than 10 mins.