ScizX and IXI FalcoN have wrote wonderful guides to getting this achievement. This cheevo is very time consuming,all in all I have spent a total of close to 6 hours getting this bugger. Here's how I did it:
Get your Silverback to LVL 3:
Mutators: Super Reload, Enemy Regen., Friendly Fire ( FF is a must, in order to damage your own Silverback)
Make your way up to the top of the tower where the silverback spawns and stay put. This is the best place to be in this whole map, you can easily bounce from one wall to the other while in cover, so use to your advantage.
Start out with wave 1, that way around wave 6 you should be able to purchase the silverback, but don't buy it just yet. I found it easiest to stockpile tons of money first then buy, damage, buy at the beginning of each wave. Once you reach wave 9 with these mutators you will encounter an occasional serapede and armored kanti every few rounds, those are going to be your biggest foes. Other than that it's a piece of cake.
I was racking up around $7000+ per wave once on wave 9. I waited until I had roughly $90,000 earned without buying anything before I purchased the silverback.
LVL 3 Silverback Unlocked:
Mutators: Enemy Regen, No Ammo Pickup, Friendly Fire (FF again a must)
Again start at the beginning, that way the silverback can be purchased by wave 6. I died a few times in the previous waves due to no ammo because of the mutator I chose, you want this. Make sure you have $22,500 in the bank by wave 6, $12,500 to buy silverback, $10,000 to upgrade.
Wave 6 spawns anywhere between 22 and 26 enemys per round. Set the silverback in the first corridor facing down the stairs from where it is purchased and just start having fun blowing locust to bits. The nice thing about this wave is it is mostly drones, grenadiers, formers, an occasional boomer and kantus, but nothing one rocket won't take care of.
Once you kill everyone except for the last guy, make sure he kills you. If for some reason he dies then wave 7 has been known to spawn grinders and maulers. These guys will tear your silverback into shreads. (Found out the hard way)
But be careful, do not waste any money on other fortifications, reason for this is because if for some reason you silveback is damaged it take quite a bit more money to repair than it does not upgraded.
Wave 6 spawns 22-26 enemies, but for mathematics sake lets just say only 20 spawn. So 500 kills needed for the achievement, 20 kills per wave equal 25 total waves. Camping in this spot means you must let the locust come to you so average wave time is 5 minutes. 25 waves times 5 minutes each, equals roughly 2 hours of play time.
Any comments to make this guide better or change something let me know. This is my first guide I have made so be easy on me haha. Any negative votes please explain why and I will address the situation.