Before you do anything change your load-out to Engineer. When in the game use your skill points to level Engineer all the way up. Engineer level 3 increases the duration of traps and level 4 increases the damage done by Traps.
Make sure you are in a lobby with 2 or 3 other people that somewhat know what they are doing.First Area
When you are taking out the hive by the Motel use the electric traps in that area, they're easy to find. You should be able to get a few kills here. If you get a hive that's on or near the road you can use those again. When you get to the "Mother Hive" there is a fire trap just past the Motel sign on the left when looking at the hive. Don't use this trap right away wait till about halfway through. Otherwise you won't get many kills and you'll just be wasting your money. You probably won't get a whole lot of kills but it helps.Second Area
The next time to use the traps is when you get to the hive on the left after entering this area, but only use the Electric trap that goes to the "Mother Hive" area, in between the 2 buildings. Most of the enemies don't come from the right where the Fire trap is, so it's pretty useless to use, but you can if you want. Now the next traps to use are the 2 that are at the short walls in the middle of the "Mother Hive" area. You should be able to use these traps 2-3 times and you should get a huge junk of the kills here. Just make sure you use your money wisely so you can use them multiple times, not very difficult to do. Stay alive and you should get this about half way through taking out the "Mother Hive" if not before.
If you don't get it here and you make it to the third area then you're on your own. I have never gotten to the third area.