If you can get a well placed plasma or spike grenade on a Brute Hero (Chieftain)'s weapon it will stick, but it will have the effects of a nade exploding near him (ie: it will launch him in the air or if his shields are down it will kill him).

I learned this helpful bit of info while I was playing the first level on Legendary. After I stuck the Hammer the Brute flew off the little bridge.

Note: I only stuck a Hammer. I have yet to try this on a Brute with the Plasma Cannon so if it doesn't work for them use your old tactics.