I didn't try them yet because the only good item they drop are dclaws and the drop chances are low.
But I got some experience with them.
First of all you need a darklight because Tormented Demons have a shield around them that you bring down after you hit it with a darklight.
Only problem is that darklight are kinda rare or just not added into shops. And you can't kill them without.
Tormented demons require the player to hybrid between attack styles because they can switch prayers.
1. I would say use vanguard it's easy to switch between melee and range
2. Bring the shield down with a darklight
3. Use the attackstyle that they're not protecting from.
4. Tormented Demons attack with one style and each time they roar (they lean a bit back) they switch their style so protect whatever you need to.
Their attack styles are easy.
If they swing their left arm to you it's melee.
If they throw some kind of rock they use range
If they burst fire out of their mouth it's magic
I hope I helped you a bit and I'll try to make a guide when the darklights are added