Hmm...maybe we can start with a throw-away villain to defeat as a good point for all of us to meet?
Example:
        The Mole Man and his troops invade the surface and we all end up working together to send him packing! ;)

Lets first ask, if Moleman suddenly invaded the surface world with his troops, tunnelling a way up (ala Underminer in Incredibles), how would we each respond.

I'll start:   Archer would grown large, and attempt to pick up the tunnelling device, and shove it back down the hole, to block the way and inhibit the troops following it.

Duelist would have leaped in to confront the troops, keeping them occupied and distracted with jabs from her sword and dodging their blows.

Kestrel would be hard to see as she turns to Air, being seen mostly as a shadow flying around the battlefield and using blasts of air against any moleman who had cover from the sky.  She would manipulate the weather so that a Lightning storm was happening over the location of the moleman tunnel, sending lightning down from the sky to strike any moleman who got past the mole machine.

He probably was happy to help Archer with shoving the tunneling machine back into its hole. All the while with a brilliant flashy light show and some loud music, something like his own version of Cum on Feel the Noise. blasting the molemen

Looking to Rock Star and Duellist to deal with this infestation,  Archer would offer her hand, as she began shrinking back down, to the flying Kestrel, “Can you get me there fast?”

As Kestrel (assumably) took Archer’s hand, Archer continued to shrink to a tiny ‘handful’ size … for easy transport.

As Kestrel came in close to the new threat, Archer called out, “Fast Ball me”.    Thrown at the machine, Archer again increased in size to ‘giant’ proportions … slamming shoulder first into the emerging metal beast.

Kestrel was about to explain to Archer that she couldn't carry anyone while flying when the woman shrunk down to a tiny doll that fit in her hand.  Tilting her head sideways, she focused her wind power out through her hand to lift the woman into the air and float her as she turned back into air.  "okay, never done this before so lets see if this work!"  By some miracle she keeps Archer floating above her hand as she flew the woman toward the other mole machine, although it took lots of focus (power stunt, might not be able to do that ingame ;) ).

"Fast Ball special coming up," Kestrel acknowledged, her voice sounding very windy as she directed her wind power to send the tiny doll toward the mole machine, knowing the woman can grow to be a giant.  Hoping that Archer times things right as she then sends blasts of lightning out of the sky and into groups of mole men charging out from behind the machine as Archer slammed into in her giant form.