Tormented demons change their attack style every 16 seconds and with this their weakness also changes. When they change style they will lean back and roar, unleashing a magic attack that can hit you behind cover.
They are also capable of using protection prayers which change to block your current attack style after you've dealt 3100 damage. This means that you will require at least two different combat styles to kill them. However, it should be noted that Tormented demons are one of only two monsters whose damage is 100% blocked by your own protection prayers.
Furthermore, these demons have a fire shield that reduces all damage dealt to them by 75%. However, this can be disabled for 60 seconds by a successful hit with Darklight.
For more information, see Tormented Demon Hunting.