The Swabbie is rather similar to the Midget Psychos from the main game. They are fast, tough, and short which can make them hard to hit. The biggest advantage you have is that you are taller than they are and they have rather large heads compared to their bodies, making it rather easy to get critical hits on them.
Like most enemies of this type, the best method for dealing with them is to simply keep your distance. Stay far away and take them out or simply back up while shooting. If they get too close, let them swing and side step to give yourself some breathing space and a clear shot.
