Skotizo

Use arclight or your best demonbane weapon. Protect from magic. Step up and attack when you see the gigantic black greater demon casting a magic attack to avoid being hit by melee. Pretty easy to avoid all damage from him doing that, so if you bring a good amount of prayer with some good food the fight is pretty easy. However, with protect from magic, his dark spawns will still hit small. 

Make sure auto-retaliate is off before starting the fight