Did this on Heroic yesterday. Took a while. Luckily both my team mates were better players than I am.
Basically we figured out a strat by the end. While the boss is shielded you can still aggro him by shooting him. Two people duck behind cover and pop shots at the boss to keep him firing you so the third can safely move the energy thing. Save your heavy ammo for the mobs that spawn and just snipe the boss. The time window is brief so don't wait to reload your sniper unless it's got super good reload, I just emptied my clip then shot with my primary. Then back to rotating, taking cover, and keeping him focused on you two while the third repeats the process.
It's my favorite boss in the game as long as I'm not with randoms