[HoneyBadger]

I was going to add this to the "Low cost tactics", but it's pretty expensive, especially for Niefelheim. Fun to pull off, though, and it gives Niefel Hags better synchronicity with the Skratti:

Niefelheim's known for a lot of things, but most of those things are huge, straightforward, and obvious. One thing they aren't known for is Nature magic, except as a path to a decent bless.

Another thing they aren't known for-being huge, lumbering giants-is being particularly stealthy.

But Niefelheim does have 1 National summons-Pack of Wolves, which is Conj3/N2, costing 25 gems for 20 reasonably tough wolves. Not cheap, but fine damage-soakers, with Forest and Mountain survival, and stealth. They're also extremely fast on the ground, at Move 28, meaning archers aren't going to get much chance at them. Not as good as flying, but not subject to stormy weather, either.
To these can be added the following units, which Niefelheim has easy access to, and which don't require venturing further than 4 levels into any magic path:

Bind Fiend-1xfiend of darkness, 5 blood slaves. Fiends of darkness are Imps' bigger, older brothers-older brothers who like to shoot steroid-pcp cocktails into their eyeballs and then mug bengal tigers at the zoo. They fly right out of the box, which keeps the enemy's mages occupied-by-means-of-evisceration, and they've got x2 poison claw attacks, which makes things more interesting for mages who only thought they'd be dealing with the cold, and maybe lightning.

Black Servant-1xblack servant, 5 death gems. Always a good unit to have access to, and a commander. They'll eat up your death gems, though, which is a problem for Niefelheim. Still, it's a great opportunity to add a few thugs with bows to your stealthy forces-but once you've got access to storm bows, give them to your scouts. They're also good out of the box, with etherial + lifedrain, and 18(!) hit-points.

Summon Shades-3xshades, 5 death gems. Expensive for what you get, but etherial, with most of the advantages of a shade beast. Shade beasts are far better, though, so wait for them if you can afford to. They do have a slight niche use as arrow soakers, since they're slower than shade beasts, and their etherialness should keep them safe from most arrows for a long time. Not really worth it though, in most cases.

Summon Shade Beasts-15xshade beasts, 20 death gems. Requires Conj 4, but probably Niefel's best all-around option. Your etherial, cold resistant Shade Beasts are fast, and amphibious, which means they can follow your Niefel Giants anywhere, if you need to team them up, and you don't have to worry about killing them with a cold-aura booster, with skratti. These guys aren't as fast as your wolves, but next to Fiends of Darkness, they're your fastest choice.

Spirits of the Wood-5xhama dryads-the most expensive option, in terms of research and path availability, but the gem cost isn't exhorbitant, and they're etherial units with natural regeneration, recuperation, and poison resistance 100%. Definitely worth the price, as a niche unit, and far better than shades. Note: They're not terribly fast, either in combat, or on the move, and I'm pretty sure they'll eventually die, if you lead them away from where they're summoned, but with some stealthy allies, they make great seige-crashers to keep around your Capital, or other major strongpoints, to get the drop on poison-using foes. They lack cold resistance, though, so keep that in mind-combines well with undead/dire wolves, and their Steal Strength weapon will take the punch out of giant-killing SCs.

These all come with Stealth, meaning not only can your Skrattis (in wolf form) lead them, you can also bring along some Scouts-which, if you're using a Prophetized Skratti to lead them, gives you instant, all-access Bless, and buffs, like Quickness and Sermon of Courage.

And later on, you can trick your stealthy units out with powerful mages and SCs like Wraith Lords, Harvester of Sorrows, Kokythiads, and Spectral Mages.

Again, it's not the cheapest way to go, or the easiest, but being able to field a few very mobile, stealthy, and quite powerful bands of bushwhackers can give Niefel's enemies a fatal surprise that they never saw coming.