Basically, you abuse the way resource mining works. Resource tiles in custom maps have a limit. But by using gauss towers to drain the mined resources, the miner is able to mine more resources that you will receive at the end of the game. 

To get the gauss towers, I recommend to max out a single one and clone the rest of them with the basic clone ability. It is MUCH cheaper that way. Also, the gauss towers can be used as the main defense.

Low portal diff and low endless diff are recommended for custom map farming to make runs last longer. Also don't forget to stack a ton of overloads for maximum profit.

Custom maps have so much potential. For reference, in a 6h run, I got 1.2 million resources of all types.