There's no communication between them and us since they teleported away, so I won't know they plan on shooting the fireball. But They should reach the room before I do, so I'll probably be entering just after they launch the fireball. If not, then they will most definitely see a room full of friendlies and probably not launch the fireball after all. It won't hurt the red dragon anyways, so they're better off just targeting the druid with stuff. With the archons concentrating their fire on the druid (or the dragon if the druid isn't within 30 feet), they will likely kill him in a single round.
So my plan is to still fill the room with archons and blast away, stealing the mirror with the one and teleporting away. First priority is to get the mirror, second is to help the sorcerors and kill the generals. If the attacks appear ineffective, or the generals wipe out more than 1/3 of the archons in a single round, then retreat immediately after getting/destroying the mirror.
EDIT: Also, if I didn't make it clear, I want to concentrate fire on one target at a time. First the druid, and if that becomes a bad idea (or if he dies), to switch to the dragon, still performing a retreat at the time specified above. In terms of attack vs. retreat, I'm trying to be as reasonable as possible. If it looks like we're winning and not taking too heavy casualties, then stay until it's finished. If things start to look bad enough, then full retreat.