Team challenges would be great, where there is an imposed party size that must be met. Give the ability to "spectate" these matches, and if someone drops out of the match then a spectator is dropped in where the other player was.

Tough team challenges would be great - however no mission should have a party requirement. I would love more really tough content to overcome, and hopefully I can get a nice dedicated team for grinchlin assault this year, because that seems to be the only replayable mission that is still super tough, and I love that challenge (first floor I've been able to clear pretty easily though, but it's the second floor that's the hardest bit anyways).

These bosses gets a very Special shield that makes them immune to any attack of a specific weapon type. One Boss will be able to have 2 shields activated and the immune shields are determined by the weapon type that did deal the most damage to him so far.

So in fight it can look like this: You needle him -> Boss gets immune to guns after the first damage number. Then you use swords -> Boss gets an additional shield vs swords, too. So what is left are bombs, until the whole bomb damage together he recived gets higher than one of the other damage numbers, which causes to Switch immunities again.

Switches for bombs only could be in each Corner of the Boss room that Switch floor traps off. One switching place is 3 or 4 Switches that must be switched in the very same Moment and each of the Switches are far enough away from eachother that no Pulsar or saradusa may do the Job.

A Switch for guns is easier: simply make one out of sword's and bomb's range. A gun Switch could prevent Shooting rockets from the side into the fighting place.

A sword only Switch would be a Switch that Needs enough damage in one and the same attack - thinking of something like These punching bags in the guild training. Needling doesn't Count since it deals multiple times damage and each single number is not high enough. Also this Special bag will instantly recover after any weapon contact - even if it does not recives damage. So poisioning or collecting catalyzer orbs is impossible.

The sword Switch could temporarily Switch off some respawnings.

I am totally Aware that this fight is impossible to be done with specific weapons and that there are definitelly 'wrong' weapon choices as the bomb Switch cant be switched with mist bombs.

But, hey, it is a Mission that is supposed to ruin Players lives and get an actually Team tactic, so it works out just fine.

Additionally to weapon type immunity, there could be a damage type immunity, too. Like immune to normal, shadow, piercing, elemental. So you really Need to plan in a Team since even with 4 Slots unlocked, you can never match the requirements to get a Boss done alone.

...then just draw a picture:

http://fs5.directupload.net/images/151106/olelshv2.png

The 2 bosses invulnerable together could be soloed using something with major knockback or shield bumping.
Players can also freeze one and have the other come at him/her.

Of course, you could always make them immune to knockback, shield bumping, and frost, but then players could still dodge, losing the aggro of both, then shoot or come near one of them to get its aggro but not the other's.