Most obviously, your big combo guy is Rook. His 'Mark Prey' ability is pure murder on Scourges. A good trick is to take as many archers as you can, armour bash with Iver, and then Mark Prey to have everyone riddle a target with arrows. Melee attacks are less effective with this ability because you will have fewer opportunities with multiple adjacent attackers.
Egil's stonewall ability works well with Alette's thread the needle; move him up to draw in attackers, stonewall, and then run her up behind and gun down a line. This is a little tricky because they don't like lining up, and she won't do much damage against uninjured Dredge with their high Armour.
Obviously Ludin is amazing against those annoying slingers because his impaling ability triggers during their defence move. If his strength is high enough a single piercing attack can neutralize a slinger. If you use Yrsa another evil trick is to push enemies back into fire patches, or shove them into base-to-base with Haakon for an easy kill.
Once you get the Mender, a nice move is to set him up so his turn is directly before Krumr's. Fling your lightning at a group, use Krumr's ability to make the Mender take the next turn and blast the same group again. The mender's healing ability teamed with Mogr's bring the pain ability is also a great way to wear your enemies down.
Oddlief's trap ability is much better than it seems at first, especially if you put the trap arrow adjacent to your warriors. When a big scourge closes in he gets hit AND HIS TURN ENDS, leaving him standing there like an idiot as you whale away on him.
Eirik is just a great all-round support character, but he's especially good when he's helping out Haakon or Gunnalf. If he gets beat up you can run him to a safe distance and just use him to lend Will to these powerhouse giants in between resting.
Hogun and Mogun's ability is useful because it makes them a threat no matter how injured they are, but it's uncontrolled and can kill enemies that you just want to weaken. Ekkill is a much crazier berserker with a hilarious effect that can line up enemies for Alette or the mender if you're lucky, but he's not as dependable as the twins.
Finally and most obviously Haakon is just a monster against clumped groups of dredge with his earthshaking axe smash. His hits will often trigger splintering in the Dredge, weakening them even more, making a powered-up Haakon one of the best units in the game.