Beating Cthulhu in Magicka

By far the easiest way is to use Propp's Party Plasma (a beefed-up version of the spell that sent you basementward during the prologue of the original campaign), you should be able to solo Cthulhu in 30 seconds using it.

If you don't wish to use the cheap method, you can do what I did:

• Stay in contact with voice chat (Pretty much a requirement for enjoying Magicka)
• Cast wall shield when he tries to suck you up
• Stay near but not in the middle of the platform, to prevent getting confused
• Use the lightning death ray spell (steam - steam - lightning - lightning - arcane) to inflict damage
• Shield when he fires purple insta-kill projectiles.
• Don't panic if you get grabbed by the tentacles, you are actually pretty safe to heal, shield up and revive any fallen friends.
• Cast shield - water - frost (ice shield) to give yourself some staying power.
• Hold RMB when you're moved towards the water, it will stop you from moving, wait it out until the purple halo around your head disappears.

If all else fails and you have people to spare, a wizard maintaining a bubble shield ready to cast revive at a moment's notice is always helpful.