On my first playthrough with my Viking on hard I used:
Demonologists
Soothshayers / Druids
Maiden Warriors
Royal Griffens
Rune Mages
I tried getting 0 Loss through the whole game tho.
Tactic been: Taking down the enemies ranges in the first round or get them in Meleerange, so they can't shoot.
Maiden Warriors move in range and use Attack Rune + Spearshot
Royal Griffins move directly to the enemies to take all the damage and deal a lot (Furious: Always retaliates against enemy attacks)
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Druids use "Train" (To tame enemy wilds); summon if you need some cannonfodder or use their basic attack at packs for great damage & massive rage-gain.
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Soothshayers use basic-attack on ranged enemies; Loki's Touch if there are non, Ice Storm if nothings possible - Waiting is also a viable option if an enemy gets low enough for Loki's Touch.
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Demonologists summons deamons (what else?
); Using their "Thread of Life" at the end to revive fallen allies.
Rune Mages usually use "Phantom" on Maiden Warriors (they use Spearshot, then use "Call to Valhalla" to revive the Royal Griffins and in their last round they "Wait" to be cannonfodder and then attack something you like to be gone soon).
If you need some blocking very soon, use "Battle Cry" in the very beginning; Move your Royal Griffins not the full range (1 Hex left) and use their "Cheer". Then you should have enough initiative with them to cast phantom on your Royal Griffins, which then move towards to unit you wish to block and cast "Heavenly Guard".
Most used spells with this build:
Summon Phoenix(More blocking / Damage / Cannonfodder),
Call of Nature(More blocking / Cannonfodder),
Battle Cry(Self-explaining),
Slow(Saving your base units from getting hit mad without having Call to Valhalla ready),
Haste(Mostly used for your Royal Griffins to engage soon),
Calm Rage (You will have LOTS),
Runic Word (For the Rune Mages),
Gift (Mostly used for your Warrior Maidens to revive allies).
Going to try something new with my Soothshayer on impossible now; Just had a lot of bad luck with the available troops on my viking. I used an unpatched version and fixed some common problems with the help of editing the .lua files
I had to use the console twice to give myself the Horn of the black unicorn (Which I didn't get after killing it) and to put the value of my rage "normal" again, after I bugged it with Calm Rage.
Going to update this post soon.