Go to wave 8 on Whirlpool map, you will have a checkpoint and a wave with only Tanks.

Make a build that the towers can deal with the tanks without your help (I recommend full Violators towers), so you can keep farming while the game it's minimized. Just remember to reload the checkpoint every time you complete the wave.

Same as above, you can do this on wave 11 on Facility map.



I highly recommend full Violators towers.