[Yrkoon]

I've had to fight helheim/vanheim in a few games and yes they are extremely strong. Not unbeatable though.

In one game, I played as Niefleheim (the giants). I had a big army of high end giants (the ones with the longswords and shield + mail armor) vs an army of vanheim (cavalry + infantry, all glamour) and lost : my giants couldn't hit vanheim because of their high defense and although my giants resisted well, in the end, they lost.
So I changed tactics : I put a few sacred giants (the ones with frost aura) in my army of giants and put everyone on hold and attack. Next battle, the cavalry arrives first on my line of giants. The giants, with their relatively high defense, take very few hits from the cavalry charge and hold the cavalry. The long battle starts, like the battle I first lost. But this time, the frost aura of the giants is there. Basically, the combat took a long time and the aura froze the cavalry and the glamour infantry that came behind and my giants did a killing. I eradicated the vanheim army and lost very few giants.
That tactic of high defense troops + aura vs vanheim/helheim is of course limited to a few nation.

Otherwise, something that works very well too, and is quite cheap, is N1 mages with high defense troops: the N1 mages are scripted to cast eagle eye then tangle vines x4. Eagle eyes increases their precision, and tangle vines (area 1, so at least 2 cavalries) halve the defense of the vanheim troops . With eagle eyes, they almost always hit the first row off enemy troops, so your units can slaughter them very quickly.
I prefer tangles vines to vine arrow because it's area effect 1 (vine arrow is 1 unit), no save roll (vine arrow has to hit), lvl 1 spell (vine arrow is lvl 3) so it's available very early in the game and very effective.