These pig-like guards are slow to move and attack, but when they swing their battle-axes they can be very deadly. Shields won't do you any good if they get close enough to attack, and they can take quite a beating before they finally keel over.
Sugggested Method of Attack - Don't get too close to these guys. Pick them off from long range if you have the time and ammo. Otherwise get in close and use the fists. Run in and get in a quick punch and then duck out of range of the axe. Repeat. This tactic should not be used in areas without some room to move or when there are several other enemies around.