Darn those X-beams!
You may be able to teleport through the beams (MMM).
Or if you're lucky, there's the "poor man's teleport" of putting the silver sapling on the other side of the beams if you can... but make sure you can get back out the same way! You will need somewhere to plant the seed on either side of the beams, and some way to kill yourself from full health, like the poison spell, tripwires, etc once you are on the far side.
I don't think there's a "remotely manipulate" spell

Worst case, you might have to abandon the quest, which I think will move the doom counter forward one.