Sen'jin Shieldmasta has very few direct counters. While the Black Knight is designed to deal with it, opponents are essentially trading a six mana card to deal with a four mana card. It works, but it's inefficient. Priests can also drop Shadow Word: Pain to quickly remove it, but it's admittedly rare to see Anduin Wrynn on the ladder, as priests are generally not favored by top players. Druids have access to Starfall and Starfire which each deal five damage to a target, but neither are commonly in use right now. Beyond those cards, there isn't really a whole lot that will destroy a Sen'jin Shieldmasta conveniently. He's a solid play against common decks like Warlock Zoo, Miracle Rogue, Druid Token, Paladin Aggro, and more.
More importantly, Sen'jin Shieldmasta boasts arguably the most fun entry sound in the game, shouting "Taz'dingo!" upon being played. If you're looking for a reliable wall that won't cost you a single bit of dust, consider tossing this troll into your deck.