I finished the Barrett boss battle (and every other highly strategic boss battle.) in like 20 seconds. Concussion grenade followed by a spam of either frag grenades or repeated Claymore augmentation blasts. Now, I forget what company Eidos Montreal "farmed out" the boss fights to, but I highly doubt they'll be getting any repeat business in that department seeing has they basically made one very generic, very lame boss battle and repeated it multiple times.