from:
http://forums.projectego.net/demon-door-combat-14-hard-663/
The absolute easiest way to get a high enough combat multiplier is to get an ebony longbow, and go to where the demon door is. You will see a shape of a troll then it will disappear (and only pop out when you get close enough). If it doesn't pop out right away, then begin to charge your bow up for exactly 2 minutes. If you hold it for 2 minutes then move over to where the troll will pop up, lock onto it, and then fire. When the arrow hits, you will kill it instantly and gain a combat multiplier of 25.
This also works with stone trolls, except you need Skorm's bow and hold it for about 2.5 minutes. You get a combat multiplier of 60+ upon the bowshot kill.