just keep kiting and picking off one at a time. or just get your stuff and leg it and go to another swamp and approach it from a blackforest so you can slowly gain ingress, never land your boat in the swamp. maybe take a hoe to try level ground so its not so deep in spots and have a more pathway made. try range attack most stuff and take care of enemies quickly as possible to avoid gangs forming. only travel during the day.