I find it unfortunate that there are no goblin regiments in the campaign (at least, I reached the Grail so far and got stuck there, too many mummies to stay afloat). I've played a few games against my brother the last few days, and I find using a goblin regiment, backed up by a mage and possibly a Horn if I can afford it, the goblins attack, then I just start firing beams through my own, cheap and expendable units to deal good damage against the heavy big guys they attack. Add in a few harder hitting regiments for an attack, and you can soften up your targets (goblin archers, fire a round, force attack, fire your beams through them, attack with heavier regiment while fleeing the archers, use horn, archers go lob at something else).
But, I think my favourites are:
Ogres, 3 in a group, cheap to replace, if you soften the enemy up they have a high chance of all 3 surviving and killing/routing the enemy.
Orc archers, who doesn't like a group that can soften up a regiment, then attack it as if it were a melee infantry unit? When using humans or undead, I always have at least one regiment ready at the side for a charge to prevent my archers getting in a melee, but with orcs I just charge the archers, then have the other regiments attack it from the side/behind or engage a different regiment altogether.
Orc/goblin shaman, I actually like the idea of a goblin shaman more. A weak, tiny little creature with devastating attacks? I'm all for it! I especially like the support role it can play, with 'ere we go' I could easily have any melee regiment defeat 2 entire, unscathed skeleton regiments and still have more then half of the guys left, if I took my time softening them up I sometimes lost no more then two guys to two whole regiments. Orcs are more about melee, and I use them as such.
I do dislike the 'Da Crunch' spell, sometimes it's effective as hell, but at one point I threw down 6 Da Crunches squarely on 4 enemy regiments (in melee combat with my fith), and I barely killed 5 guys with it, even though my brother claimed there were no anti-magic items/spells at work at that point in the game, and none were visible.
Bright mage, seriously, that flaming head it TOO GOOD. Every unit you get under that shrieking monstrosity is almost guaranteed dead, making even the best regiments flee. If necessary, I just put my mage in the way of cavalery, ram one of these heads in it, two if I get the time, then just teleport away. If I don't just teleport behind/at the flank of an enemy regiment and burn an entire row+all suckers that move into it's path away. If it were up to me, teleport would get a maximum range and the flaming head would get a max killcount.
Trolls, there is something about a massive towering unit that, with a little assistance, can beat an entire regiment, then just heal up to full again and attack the next, all the while able to block some magic attacks.