(Init of Skeletal rust monster:2!) The monster clearly sees the pair of you and you decide it best to quickly exit the room again. Dragnar is almost too slow, Reece had to drag him out so he could shut the door before the skeleton crashed loudly into the door from the other side. (Break door:3!) The skeletal thing went silent for just a bit before beginning to mercilessly attack the door, ramming it with it's thick head. The booming sound echoed through the stairs and beyond and you can almost hear the claws of the creature grating on stone. The door holds for now, giving you time to prepare a little. They lock, of course, but neither of you has the key so Reece is doing his best to keep the door shut. Fortunately it opens into the room and the creature is having a hard time with it.
With every hit the lock trembles as Dragnar does his best to figure out how to prepare his spell to 'weaken metal'. (Runesmithing:6!,6!) In a flash of insight it comes to you and you realize now what you mentor was talking about, using the runic structure to define the constraints placed upon the metal. If you just invert the first, third and seventh runes in the sequence you should be able to invert the intent. You quickly sketch out the runes with your fingers, speaking in the ancient tongue as the silvery runes come to life in front of you. You hold them there, with some effort and wait for the creature to break through.
The pounding is somewhat anti-climatic. It looks as though it is breaking the door slowly, but it is taking time. Had the creature not already been dead it would undoubtedly not be hitting the door as hard as it is though. After nearly half an hour or so of tense waiting you hear the lock give way. Reece continues to hold the door closed, and since the creature isn't pulling on it, doing so with little effort but with every hit the door bounces some before he can close it tight again and you begin to see small cracks forming near the edges.
(BTW this is a classic DND rust monster, which does exist in a variety of games, but the touch of the tail is technically what rusts metal. All metal, though some resist it better. The creature then feeds off the residue of the metal. It is considered bestial, hence the now undead state.)
Reece, you're holding the door closed, Dragnar you're maintaining a spell. Dwareet'ik has the option of rejoining you for combat this upcoming round. List any other preparations you want to take. Closing the door did buy you some time and if you wanted to run you would still probably have a minute or maybe five head start if you wedged something underneath the door.