Rest Ice Compression Elevation?
Archers don't need an RoIce for the Tower, they just need a blanket, lots of PC rocks for everyone else, a couple (lets say 4 or 5 just to be safe) of (+1,+0) bows, and a means of disabling the wyrm, which everyone should have, and some arrows
Once the wyrm is stunned (of THUNDER) or blinded (of DARKNESS), normal 1d6 arrows are enough to kill it, although slaying ammo is faster