Select Offline>One Player>Campaign>Bots Off>Normal
Then go to chapter 1: Mission 4 "Ditchslap"
You will spawn and immediately have 3 Mechs right in front of you (large yellow robots). Get in the first Mech and take out both parts of the bridge in front in you. Hold down RT to shoot your rockets the entire time, it does not overheat and feather the turret (LT) whenever you want but it will overheat and need cooldown time. After blowing up the bridge look left as a horde of spiders and ants will come at you. On normal and with the Mech you should have no trouble with them. Keep moving around but stay around the area you spawn at. Off in the distance 2 small Hectors will walk toward you. Start shooting them immediately aiming for their chests. (They will also drop weapons) Once they are dead more bugs will come including ticks just keep moving and shooting rockets. When and if your Mech is destroyed simply get into one of the other two that will be available. An ant hill will appear toward the direction the hectors spawned. Eliminate them easily and now you can do one of two things. Patiently wait for more ants to spawn out of the hole or get out and blow it up (getting you one closer to that achievement). I did the latter. To be safe and make sure that the kills would count, I looked at the ground with my rocket and shot until I was dead. Then you can choose to either quit and check your progress or retry.
Using this method I was able to get roughly 200 kills every 8 minutes.This is not a great method for farming kills however kills effecting other achievements break down as such.
1) Ants 90-120
2) Spiders 50-70
3) Ticks 15-25
4) Small Hectors-2 Plus weapon drops
5) Blowing up the ant hole