Usurper counter is another Usurper.
Pitfall trap completely counters Letho and Viper witchers cards.
So the counter is to play the same deck (can't imagine anything more boring) or to play ST because there you can include one Pitfall Trap that you will play just before your opponent plays Letho and VW? Thanks for proving my point.
Countering a strategy means anticipating your opponent's deck and strategy, building a counter deck and playing it properly. If you expect your opponent to use big boosts, then you counter with a deck that has resets and big removal. Usurper, Letho and VW just mess up the basics and essential mechanics (leader ability, mulligans, key cards) of any deck. That is not strategy and certainly no counter. That is broken.