My advice is to use everything in small doses. There are so many ways to interact (intentional design of the system) with another combatant, that everything should be sprinkled with equal measure.
Have a necromancer at the back gating Seith Aberrations, while warriors keep her protected. The players get to see their PF and Parry work well against the physical attacks, but they know that they need to break through sooner rather than later in order to deal with the Spiritual damage source.
Think of the following as a good checklist of effects, and try to use every one before you go back and use something again. Variety adds fun, and puts forth interesting decisions rather than just crunching numbers.
- P damage
- M damage
- S damage
- Degeneration
- Blind
- Impeded
- Vulnerable
- Aura
- Taunt
- Rage opponents
- Shroud opponents
- Area effects
- Range-ray effects
- Ranged combat
- Melee combat
- Swallow (effect)
- Super mobile opponents
- Mounted opponents
- Hindering terrain
- High Healing opponents with Stitch Kindred
You can also add more variety by introducing variable combat terrain. Sometimes make it close quarters, other times wide open. Play with light as well, making areas which are visible, and others that are complete darkness... it's Fimbulwinter after all!

A lot of RPGs have interactions limited only damage, but here you get a lot more options for interactions.
Remember to reward them good team work. They shouldn't feel punished for great synergies between builds. It just means that you need to bring your A-game, and we're here to help.