My ideas so far(Sawbones would voice them once other people would post, since it's impolite to run ahead of them) are:
Sawbones and Nergal in the lobby/on the stairs, keeping up the gas cloud and bonking anyone who tries to push through. Area denial.
Mako on the second floor/in the back of the first floor, ready to zap people with his trident or, in critical case, draw the water froom Maine to facilitate our absconding. Fire support/flanker(ready to pounce on someone trying to flank US).
Phone Freak and Black Magic stairdancing and either supporting other groups or slinging the spells at the attacking force to keep them pinned. Fire support/battlefield control.
Ed, depending on if he's packing sniper rifle, on the top floors or with mages, keeping the heat on the attacking forces from the windows. Fire support/pinning and critical assets hunting.
That leaves us with the Hannibal, Maverick, Queenie, dragon and the Harpy in reserve. Depending on the attacking force, we might have to knock their fliers out of the sky, so there's that. Otherwise - Hannibal, Maverick, Queenie and Harpy cover our shooty guys, dragon joins people slinging stuff at the attacking force from the windows. Close quarters cover and bogging down/Flying flankers.
Now, this plan assumes those guys would not have reasonable protection against toxic cloud created by Sawbones and Nergal - if they would have, then we'd have to defend the lobby conventional way or abandon it altogether and risk people going for the vault, but otherwise plan stays largely the same.
Again, the plan assumes that there would be mostly police forces; we might need to change our plans based on what supers exactly would show up.