Buccaneers are an entirely new enemy that is somewhere between the Corsair and a Goliath. They are rather massive in size and carry two weapons but the two weapons are swords and they prefer close range combat. They can be fought similarly to most other close range enemies by backing up but they have a couple of tricks that can make them a little more difficult.
The first ability, which allows them to deflect bullets by swinging their swords, feeds directly into their second ability which allows their swords to absorb elemental properties. For instance, if you fire a normal damage weapon at the Buccaneer, he'll stop moving and start swinging his swords to reflect the bullets towards you. If you were to shoot a fire based weapon at him, not only would he reflect the bullets back at you but his swords would turn red and temporarily be capable of inflicting fire damage.
Since he stops to block attacks like this, it can be manipulated to suit your needs. If you shoot his torso a couple of times, he will stop, allowing you to get away from him and into cover. The first few shots generally hit which makes playing a hit and run game profitable for you. You can also go for his legs and back to avoid this all together. If you happen to be sniping, make your first couple of shots count so that he is dead before he manages to bring his swords up.