Brutes are Extremely big and powerful creatures in the Halo 2 and 3 campaign, they are not found in Halo 1 or 4 for reasons I cannot say. On legendary they are equally or more fearful then the Elites.
One of the qualities of the Brutes is if you enrage them you will know the definition of APE SHIT. To obtain this achievement you must kill 100 of them, luckily on any difficulty. Playing Campaigns Halo 2 and 3 on easy will make this much better. I recommend finding a spot where there are a good amount of Brutes while you have a shotgun or a Rocket and continually killing them, then killing yourself. Repeating the process until you unlock this achievement.
I have found a educational and funny video for you to learn and laugh at the brutes. Video was done by Machinima.