use the Shotgun more often, and keep an eye for the ammo because you can carry limited amount of shells
switch to pump action shotgun whenever possible it is quicker and carry 8 shells instead of just 2
this is an easy achievement and you can get it in your first playthrough so there is no need to rush or reload sectors as long as you use the shotgun whenever you have ammo
just a tip : to conserve ammunition try to get close to any unfortunate taken you're aiming at, it's much deadlier at close range, so if possible take out their dark barrier and get close, then shoot them with the shotgun for maximum effect, it is also possible to kill two with one shot which unlocks another achievement