As with always, don't farm solely for the enemies, tie it together with something else.
My suggestion:
Tie this achievement as well as the 5000 ticks and getting enough cash to buy everything together with trying to farm all the weapons.
For this, you need to have beaten the game at least once (on normal, for example) and leveled one of your armours to Rank 8, thus having gotten the Pesticide weapon of that class.
Now, you start up either 3-2 or 3-5 in remix mode and inferno difficulty. There will be a ton of spiders and bombers, netting you about 8-12 weapons each time, and a couple of dozen kills in each section - and your blast will wipe them out pretty much in one go.
Each run takes about... 5-10 minutes, depending on your tactics and skill of not blasting yourself up. It is highly recommended to finish the levels for the needed money, too. You'll make about 25k in 3-2 and something in those lines in 3-5.
If you find 3-5 too hard, 3-2 is really easy to pull off and has no real tricky part if you always keep distance to everything.
Alternative Tie-in with 10,000 Survival kills + 500 Waves.
Take Marooned, the second wave has about 26 spiders in it - the third one has gun-ships and wasps.
You can also combine both. getting my necessary 7000 kills ended me up with about 4000 ants kills, 1000 spider kills and about 2000 gunships on that map, as well as about 100 rounds..