[Wolfen at the Cave]
The black beast will shift its head out of the way of the axe that Pathfinder threw, the distraction working. Waterstrider's axe will strike close to its intended destination, but slightly off center due to movement, and will impact with the muscles of the tail, chopping right through the creature's scales.
and then Pathfinder lands on its back, and it goes into complete and utter panic. it's back is it's one blind spot, and it won't tolerate something being up there.
With a roar of pain and anger, it begins thrashing wildly. the tail is limp, but it still carries weight as the creature's flailing moves it around - Waterstrider may want to get himself clear before the random movements cause his doom. For now, Pathfinder will find himself dealing with bucking and spinning.