SANGHEILI! (Oh, wait, what did I say?) Never mind. It depends on how many of each are there and what class the are. Sangheili are fast, vicious, and have Energy Swords while Brutes are strong, burly, and have Gravity Hammers. If it is one-on-one, (let's say Zealot vs War Chieftain) the Zealot would probably win because of its speed and Sword. The Chieftan's Hammer is just too slow, and the Sangheili is small, thus enabling it to dodge the majority of the Brute's assaults.