It's not that hard with a team of 5 that communicates. Farm each level for as long as you can by having the last target be a "hero" cog and then downing them and letting them get back up as long as possible each wave. Savage drones work well for this as
the others are wild tickers, destroying everything. Do this for 4 waves, and then move on to 2 boomers, 1 kantus, a drone and a wild ticker. Repeat until 10-12 in which you should ahve enough money to go beserker each wave.