How to keep HRE at bay as a pagan (Old Gods)?

In Crusader kings I am playing as Pomerania and as a matter of making my game more interesting I decided at the start to not convert to Catholicism and stay as a Slavic pagan until the end of the game. I have managed to cut Great Moravia down to size. However in 10th century HRE formed out of East Francia and controls far more than its historic region - they control Italy, South France (Occitania), Germany and Lotharingia. So their manpower is about 150k, mine currently sits around 45k-55k. None of the larger neighbours of mine are of Slavic religion (Suomenusko to the east, the rest Catholic), so no feasible alliance marriages outside the realm. I control almost exactly the area of the in-game dejure Wendish Empire (although I have not formed it). Also, I have reformed the Slavic faith.

In this context, HRE declared a holy war on me for one of the duchies in the west. With the difference in manpower and their humongous stacks, I stand little chance of victory in the conflict. Short of accepting the conversion to the Catholic faith (which ends the holy war), what are my short and long term options at keeping the HRE at bay?

Such a vast empire is bound to get split at some point, particularly after succession. You can either force it by causing.. an unfortunate incident, as Kyle suggested, or sowing dissent with a powerful king-level or powerful duke vassal of HRE who dislike their current liege.

Of note is that you can beat larger armies than yours with some clever warfare tactics. You are a pagan, and if the enemy hasn't researched Military Organization at least two or three times (unlikely, if you are that early in the game), then they suffer severe attrition in your territories. Also important is that hauling such large armies can take time, and would give you a few precious months of consolidating your troops and destroying some of their armies while they call their Occitan legions. Hopefully your realm is much more compact than theirs, and have ample naval access for quick transport on sea rather than by land.

If you deliver significant damage to them, factions within their empire might grow powerful enough to cause a civil war. This can give you a significant advantage and might cause them to ask for white peace to deal with troublesome vassals. This will give you some ten years of breathing room for you to expand and grow until they are able to strike again.

As you always have a casus belli against them, yielding some lands to them now without a fight might be a good longer-term strategy to reconquer them later when they are embroiled in another conflict -- particularly if you don't hold any of the counties in the duchy.

I am afraid your situation looks grim. It would seem that something like Paul mentioned is best. Perhaps try to assassinate the current Emperor. If he has young heirs, this may be enough to begin a civil war that will occupy his attention or cut down on his forces. My experience with using your spymaster to lower relations is that it does not work fast enough. Or as fast as a manure bomb.