I like to hold the F key to auto-attack a single enemy encounter. Holding F is pretty useful against the shadow creatures because they sometimes teleport on top of you, and you get a free hit on them instead of them getting a free hit on you. The problem is that auto-attacking Swimming Horrors causes a stunlock. They take too long to teleport away so they get hit again, which causes them to try and teleport again. This can repeated to cause them to be unable to teleport away, however they take no damage from this. It kind of just bugs up an otherwise smooth fight.
Hold F while attacking a Swimming Horror. Unlike the other shadow creatures, they will be stuck and unable to teleport away. They are immune to damage during this though, so it is just a waste of time.