If you are playing on the Xbox 360, you need to kill 100 players on PC.
If you are playing on PC, you need to kill 100 players on the Xbox 360.
Most players are on the Xbox, so you can get this achievement easily by getting the PC version and playing on that. Then simply sell the game on Ebay or something if you don't want two copies of it.
Both platforms share the same achievement card, although I'm not sure if statistics and cumulative achievement progress carries between the two.
No matter what, you will get this achievement eventually. If in boosting sessions, try to determine if there are any people on the other platform (ie, PC) that you can kill.
NOTE: In every game you score a kill, there must be at least 4 human controlled players on each team. This is for every Shadowrun achievement.