Greater Demons are a Mid level demon that can be taken down by using ranged attacks, you can use other methods but ranged is the most efficient.
Greater Demon Location
The best place to farm these on a slayer task is Kuradel's dungeon. Tele to the Kuradel Slayer Master and then go into the dingeon there.
They are agressive, so you can mostly AFK them there and they dont seem to do too much damage with t90 gear on...