The only place free-to-play players can kill greater demons is in the Demonic Ruins. Since this is a multicombat area in high-level Wilderness, there is a high risk of getting killed by other players. Because of that, the best strategy to kill these is to bring as few items as possible.
To do that, first train your Prayer level to 43 to use Protect from Melee. Since the Ruins slowly restore your prayer, you can use this prayer permanently without draining your prayer points. Simply bring a weapon (for example a rune scimitar or Silverlight) and an amulet of strength or amulet of power, a Mooleta, and nothing else to maximize your damage in combat while not having to risk losing items to a PKer.
Additionally, there are multiple safespots at the Demonic Ruins from which a player can use ranged attacks to easily take down a Greater Demon within a few minutes of sustained ranged attacks.
Brimhaven Dungeon: Standing at the end of a vine just north-west of the stairs, the fungus west of the stairs, in the upper level at the end of the path, and at the farthest south-east corner of the plateau.
Catacombs of Kourend: under the arch which divides the Greater Demon area, and at the extreme north-east corner of the dungeon behind the rope exit.
Chasm of Fire: behind the many pillars and narrow pathways.
Entrana Dungeon: behind the burnt corpses (use the second square south of the stalagmite and two squares away from the sitting burnt corpse).
Wilderness Slayer Cave: stalagmites at the entrance of the Greater Demon chamber, halberds may also be used.
Isle of Souls Dungeon: behind the two rocks on the Eastern side of the inner wall, south of the skeleton and demonic symbols.
Ogre Enclave: in the south east corner of the dungeon are 4 greater demons trapped behind a fence. (requires Watchtower quest to be completed until the point where you can access the enclave)
Level 92, 100, 101, and 113 drops
The following drop rates are provided by Jagex, unless otherwise specified.
^ 3.03.1The talisman dropped depends on where the player is located (map); typically, a nature talisman will drop above ground level and a chaos talisman will drop underground.
^ 4.04.14.2Only dropped if Legends' Quest has been finished; if not, a talisman is dropped instead.
The following drops are only accessible while killing the Catacombs of Kourend variants.The drop rates listed scale with the hitpoints of the monster's variants.
^ 1.01.11.2Players will receive the totem piece that they possess the fewest of. In case of a tie, the pieces are prioritised in the order of base > middle > top.
Wilderness Slayer tertiary
The following drops are only accessible while on a Slayer task given by Krystilia.
^5x more likely to drop when the corresponding sigil is active and the kill condition is met.
^ 2.02.12.22.32x more likely to drop when the corresponding sigil is active and the kill condition is met.
Contracts' drop rates are increased when a demon is killed under a particular condition, depending on the sigil above its head. See Chasm of Fire § Sigils for more information.
The Slayer Rewards interface for the Greater Challenge unlock depicts the head of a demon wearing a spiked collar. The only demon who wears such an accoutrement is the one that can be "built" in a player-owned house.