Bruisers are a cross between Psychos and Marauders. They are intelligent and capable of using ranged weapons but they are also strong enough and tough enough to get close. They are easily distinguishable becaues of their large size and shirtless bodies coupled with holding a gun.
While they are capable of both ranged and melee combat, they don't excel enough in either to cause you to change your tactics most of the time. Stay just out of their effective range and they'll be reduced to ambling after you at a slow pace. At this point, take your time and make sure your shots count. At this type of pace, it should be pretty easy to get a headshot compared to some of the more agile enemies (like psychos or even Marauders who are prone to ducking quickly).