Fenyang is one of the apprentices to Remuses and is found within Black Castle's "Halls of Havoc". After passing the first Black Castle challenge (See: Black Castle) take a right, clear some adds, and pass the second Black Castle challenge. Go up the stairs and you will reach his chambers.
Fenyang has a fun strategy, which done right can make him very easy. Make sure you clear the room of all adds first. Have one tank, one healer, and two dps attack straight at Fenyang(Varies according to group). You will need two ranged dps (melee CAN do it but it is very hard for them, so ranged are preferred) to stand at each of the two doors, killing all the Acolytes which enter the room. If an acolyte reaches the center they will pause a second, and jump into his massive pit. Each acolyte that jumps into the pit will heal Fenyang for a large sum(Around 20%), luckily they are very easy to kill.