I love these beasties in that they can quickly turn a tromp threw the swamp into a frantic search for a party member that has been snatched up and carried off into deep dark muddy waters. It's almost like jaws, you can be walking along on dry land thinking about a cloud formation in the distance and *snap* get snatched up.
The only way to prevent an ambush like this is to chain the party members together, with yes, a chain. That or have at least one party member who is very good at spotting. Other than that I think just a strait up attack on the thing is most effective, preferably on dry land.