Barrett is the largest of the three augmented super soldiers that you will encounter and can be a pain if you don’t act quick and use the room’s explosives directly on him.
His main attacks are to fire his heavy machine gun for a very long time, whilst moving towards you, and to throw out a selection of grenades to force you to change position.
If you need ammo or weapons:
To the left of where you begin there is an alcove with some guns, ammo, and grenades sitting on a box to your right, this should be you first port of call. The room’s opposite corner also has a few goodies if you need them, but your best form of ammo are the explosive barrels sitting around the arena which you can pick up and throw.
Taking him out:
Barrett is a huge hulking tank of a man, but isn’t too fond of any sort of explosive.
Start the fight by instantly taking cover, as he will spare no time before unleashing a volley of gunfire on your position.
If you have a frag, gas or emp grenade or mine handy, lob it at his feet to cause a staggering explosion. If you don’t, you’ll be running left into the ammo alcove straight off.
Once Barrett is stunned run at him and unload some bullets into his face. Not for too long mind, after a short burst dash right and you’ll find an explosive barrel, grab it, spin around, and send it into Barrett’s face to cause massive damage.
This won’t stun him mind, but he normally goes into a bit of a grenade spree now so either go for another barrel (they are all around the room) or circle around him unloading some lead into his face whilst moving for a good cover position.
If Barrett is still standing after all that punishment focus on trading a few shots with him, keeping a pillar between you and him if his big feet start stomping too close. He should fall down soon enough.