The Knight can only target one target at a time. Using cards such as Guards or Skeleton Army will distract and overwhelm the Knight. Keep in mind that his rather high health allows him to plough through smaller swarms before they can destroy him.
He can survive a hit or two from a Prince, although swarms are still recommended over a Knight.
A Knight can be used to bait the enemy's swarms or Infernocard for your larger units.
He can be supported with Skeletons, Goblins or Spear Goblins to deal high damage quickly. This combo is not only cheap but versatile. It also gives you enough time and Elixir to defend against your opponent. Deploy a splash or air troop like the Bomber or Minions to counter one of these pushes.
It's very effective to use him against support cards such as a Wizard or a Witch due to his fast hit speed, high health and cheap cost.
The player should not ignore the Knight since he can actually deal great damage to a Princess Tower by himself. Use cheap troops like Skeletons to pull him towards the center so that the Knight gets distracted and can't get to the Tower easily.
The Knight was released with Clash Royale's soft launch on 4/1/16.
From the Knight's description, the Knight and the Barbarians are said to be distant cousins, due to their physical similarities and especially their mustaches. In fact, they do the same damage per hit as mentioned above, but the Knight has a faster hit speed, so he has a higher damage per second.