Your initial thoughts are correct about the armor. This link describes how to kill a brute and offers a brief video with some general info.
Brute's defenses consist entirely of a yellow Armor bar. To deplete this you can use fire-based attacks and Powers meant to reduce armor. The most important thing to remember is to keep your distance. Stay behind cover and roll out of the way when it nears you. If it hits you, your shields will evaporate. Hit it with Grenades and other explosives. Armor Piercing Rounds and Disrupter Ammo are good to help reduce the armor, but explosives can momentarily pause the creature, giving you valuable time to get away.
The other thing I've been doing a lot is saving my grenades for large enemies and bosses only. I've really only had to deal with brutes and atlases so far, but having grenades while your other abilities are in cooldown makes a big difference.