Hard Eight and Hunting Party are my favourite of the mutations. Both pretty challenging.
Hard Eight doubles the spawn limit of specials from 4 to 8 and decreases the spawn timer. So shitlots of specials. Very hard.
Hunting Party just has absurd numbers of hunters. Also very hard. Really good training for dealing with hunters though (i.e. learning the timing of the push when the pounce). Play a few rounds of this and you'll find yourself getting pounced much much less.
My favourite mode is just Expert Realism though. Challenging as all fuck and really *requires* co-operation, which is the great part. You *have* to work together.
Whether to go magnum or melee is a tough choice. The magnum is very powerful on <= advanced mode and, as you say, having the magnum while downed is a big deal. On the other hand, it's pretty useless when you're swarmed. Melee weapon is great when swarmed (pretty much its only use) but means you have shitty pistols when downed.
On balance I think magnum is better on advanced or lower but no real use on expert or higher.
The chainsaw's a shit weapon but a good decoy. If someone goes down during a rolling crescendo and they're hard to get up because they're swarmed, a third person can use the chainsaw to either clear out or just attract the zombies away. It doesn't really have much use aside from that though really and a pipe bomb is better in that situation (as are bile jars but only if you're not going to need them later (dead centre, carni, parish), they're kinda precious on some maps).
I'll definitely be up for playing some this weekend!
Must try those mutations. Haven't tried expert mode yet, still trying to work up to advanced ( is there something harder than expert?). Yeah I should have a better idea about our plans for the long weekend by tomorrow night. I'll let you know.
Advanced is only *marginally* harder than normal, you should be fine on that.
And yeah, Expert Realism is the hardest mode. Realism is a mode that can be applied to any difficulty and it changes a few things - the player highlights are gone, witches kill you instead of downing you and zombies are more resistant to non-headshots. It makes Expert mode significantly harder (mainly due to the first one - the lack of highlights). You can then, if you're fucking mental, apply Expert Realism mode to a mutation like Hard Eight.