Knights should attack them as soon as possible with a two handed weapon since they are weak in Melee and defense. Make sure you trap them in a corner, they can be a real pain when they are running around everywhere. When blocking Infernalists it is normally more efficient to use full offense rather than full defense. Running in their direction and using Groundshaker is a good way to make them visible again, you can also use Whirlwind Throw shortly after they go invisible and they will be visible again. A Paladin could solo a Infernalist by trapping it in a corner and attacking with Assassin Stars while using Divine Missile to break their invisibility, alternatively Divine Caldera if the Infernalist isn't trapped.