Commander Niall is an optional boss and is located in Castle Sol, which is located in the Mountaintops of the Giants. The enemies here are tough, but the area isn't too large which is helpful!
Niall will summon 2 knights to assist him in battle, when you kill them they are gone for the remainder of the fight. Summon your spirit ashes to help you deal with Niall and his 2 knights.
Once you have defeated the 2 knights, you will be against Niall, he is very strong with frost and lightning attacks. He will kick, slam and sweep you throughout the battle, he is very tough!