The best way to unlock this achievement now is done on a sever with some mods.
You will want to set a server to high drop rates for everything so you can get easy access to weapons and ammo. It would also help to set the xp to higher modifier as well so you can boost up your stats for more damage and perks.
Once done all you need is a few buddies to set up some beds nearby and just like Danny DeVeto just start blastin. The faster they can click revive the faster you will be able to boost up some kills.