It's difficult to strategize when I still don't know how wide the pass is. Can you post a link to this RP or giving us the info that you have? 

I don't know how steep the slopes are. I don't know how high the mountainside goes. I don't know the terrain. I don't know the enemy's composition or position. I don't know if the GM is able to whip out a super convenient item whenever he feels like it (or if the urn is singularly unique). I don't know the effective range of the urn. I don't know if your goal is ultimately offensive or defensive. I don't know how powerful your magic can get.

Because I don't know these particulars, these suggestions may be dumb:

That Urn has Got to Go
  Do whatever subterfuge you need to steal or destroy it.

Disrupt their Supplies
  Send out spy-mages on covert missions. Climbing up or down the slopes, circle around to their camps, behind enemy lines.
  Block off their supply route with a landslide or an avalanche.
  Poison their food sources.
  Set fire to the forests on their side of the mountain: no local supplies for them.
  Shapeshift or use the armor of fallen enemy to sneak into their ranks for recon and sabotage.
  Give their warhounds rabies, strike them with madness and let their own people deal with them.

Make Pass Impassable
  Use mages or explosives to destroy the pass
  If the pass is "too large" to destroy outright, you could at least constrict the surface area of the battlefield to make it easier to deny flanking maneuvers and mitigate their advantage in numbers.
  Bonus points bring an avalanche directly into their forces, or to crumble away the stone beneath them.
  Create massive ice slicks that slope towards the cliff edge.
  Or just use ice slicks to restrict their cavalry.
  Dig out a trench that spans the width of the pass, using temporary bridges until it's done. Then fall back, removing the bridges. 
  If you've burned down the forests on their side of the pass, they won't be able to build proper bridges any time soon.
  Booby trap the trench for extra points.
  If the situation is truly un-winnable, your best bet is to find some way to destroy the only avenue they have of getting to you.


But yeah, this RP actually sounds very interesting. I might have to rip off this idea.