To get "Stalker" you must get 50 trap kills. And yes, these kills are accumulative, meaning whether you accomplish kills during separate campaigns or die after the kills were made and never reached a checkpoint, the game will still keep track of your total trap kills.
I have a quick solution. It is for those who subscribe to the run and gun philosophy, or those who simply don't want to take the time to lay traps and throw rocks at enemy's who are dumb as nails.
At the beginning of the level called "Mines" in the first campaign you have just traveled down an elevator shaft and are followed down by two headstrong enemies who are taking the stairs. If you are playing through the campaign you should have a full compliment of explosive traps at your disposal. If you simply loaded up this level there is one sitting right outside the elevator to the right. As you exit the elevator you should turn right and head up one set of stairs to lay the 1st explosive, then lay a 2nd explosive at the bottom of this set of stairs. Step back a safe distance and await your 2 trap kills. Kill yourself and load the last save, then repeat.
SIDE NOTE: The game never shows you how to lay an explosive trap, so for those wondering.... It is similar to setting a tree branch. Press RB to toggle to your trap. If you look at the ground you will see the potential explosive. Go to your desired spot and hold RT until the trap is set. Then back away. You can set off your own traps!