Use this monster as a looter. Remember looters loot resources 4 times faster than other monsters while attacking. (If you haven't unlocked this monster, you can use Bolt because he is also a looter.)
Brain's greatest asset is his ability to target lootable buildings while looting 4 times faster and 20% more than other non-looting monsters. This means that all its attacks will solely gain loot while other monsters can be distracted by random nearby buildings and the occasional tower inevitably, leading to time lost and damage.
Brain's Monster Lab ability (Invisibility) allows him to attack when one or two towers are still up. An advantage of the Brain over the other monsters is that it will only attack resource gatherers until there is nothing left. Remember, Brain does not completely destroy Harvesters.
At the lower levels, the usefulness of Brains are questionable: they require 12,000 goo while doing only 100 damage per monster - too slow at taking down buildings. To use them effectively, they should either be massed in large numbers or sent in with damage support from high-damage monsters. Beware of the laser towers or your Brains will be like this...Many people think Brains are weak monsters that are only good for attacking, but they are actually very good in bunkers with the Invisibility ability. They will immobilize the attacking monsters while the towers attack. It is necessary to use other monsters as tanks and minesweepers so that your Brains can be used to its full extent, since Brains have low health and still trigger traps even when invisible.
An example is using Crabratrons or Ichis to tank the damage and a Fomor to buff your monsters while the Brains steal all the resources from Harvesters and Silos.