When there are hobbes in the area (before the trolls), use the Slow Time spell, then the Force Push spell repeatedly, even when the hobbes are dead. The combat multiplier will rise until their bodies disappear. However it is possible to do this without the Slow Time too, in this case you should kill them close to each other, so the Force Push can reach all the bodies.
If you have Skorm's Bow, enter and exit the Greatwood Caves area until an Earth Troll spawns. Stand as far away from it as you can so it hides underground.You can also stand behind the boulder closest to him on the outside of Hobbe cave. With your bow, hold the fire button for at least two minutes; the longer you wait the more powerful your shot becomes. Walk towards the Troll to wake it up and hit it with the powered-up arrow. With any luck this will cause a 1-hit KO and you'll have a high enough combat multiplier to satisfy the Demon Door.
Another option is to repeatedly kill the troll, enter the Hobbe cave, and return. Eventually, doing this without getting hit should get you to 14.
One other method is to use the guild seal to teleport from the front of the Demon Door elsewhere and then fight your multiplier up to a high value (28 or more). When you recall yourself to the Greatwood Caves, your multiplier will be halved, and you can immediately speak to the door.
Another option is to simply use the shield spell in the Hobbe Cave. Just make sure you have plenty of will potions.
In Fable: The Lost Chapters, you can use your summon spell to create enough monsters to reach the needed multiplier.
If you level lightning up to level three, along with physical shield up to at least level two, you are able to build the combat multiplier easily. Firstly, bulid the mulitplier by eliminating all the hobbes near the Demon Door, before quickly running into the Hobbe Cave. Build up your multiplier in there by zapping as many Hobbes as possible, till your combat multiplier reaches level 20, then run back out the entrance to the Demon Door again. Your combat multiplier will drop to 15, but you can zap the rest of the Hobbes to build it back up.