Best way to Boost the kills in this game if to find a dedicated partner since you will spend about 18 hours total to get the achievement for bot of you that 9 hours each to get the 2500 but for the 500 it should take 3 hours per person and the 100 kills wil take 30 mins
then set up the game wit these settings
Map: Lab Rats
Time: 30 MIns
By setting the score to 0 you can obtain about 150 kills per match by doing it this way you also wont have to play as many matches
Also for the weapons each player you should both choose the gewehr sniper rifle its a one hit kill with a head-shot and if you both pick it you will have plenty of ammo lying about.
Leaderboards do not track your kills accurately so try to keep track yourself if possible
Good luck on this time consuming achievement.