On wave 4 or 5 you'll get 4 heroes to defeat. Since you need an execution move, you can down them with the drones but not kill them. You get about 3 seconds added to your clock for every down plus $100. Therefore, if you can get 4 heroes downed (they then recover in about 6-7 seconds) over and over, your time starts to increase and money goes up forever. We increased the time remaining from 1 to 3 minutes before someone accidentally killed 2 of the 4, but if you have 3 heroes remaining, this may still work. I'm only 100% certain with 4 heroes.
Keep in mind, you need 3 or 4 players given it's hard to manage all those downed guys and you'll run out of ammo before it auto-replenishes. It's also a good idea to have someone use a ticker (wild) when you have theme downed to take down the fences. The heroes can crawl behind a fence and mess with your plan.
It's also a good idea to have one person be the Kantus too for health.
Anyhow, we just found this out today, but I had to stop before beating insane mode. The money plan above took about 40 minutes before we thought we all had enough to win easily, and I had to drop.
Anyhow, it's sometimes hard to round up 4 players, so feel free to send a friend request and we can get the achievement... My live account ID: JT360nWiiPlaya
