The PCs and the solars are no-brainers. Between the beholders and the dragon, I had to think a bit, but I came down on the beholders.
The first thing is to get airborne, at least 125 feet up (out of kobold sling range). The PCs and solars can teleport me and the beholders (yes, beholders fly, but they're way too slow).
At this point, only two threats remain that can reach us: The dragon and the storm giants. 8 beholders' worth of eye rays will burn out the dragon's LR and annihilate it in a round. Their anti-magic cones point downward to shut off storm giant lightning bolts. The PC wizard will put me in a forcecage for safekeeping.
The solars, meanwhile, rain arrows on the giants. It will take them a while with the anti-magic effect in play (disabling the arrows' magical properties), but the beholders can always adjust their AoE to leave one giant exposed.
The PCs stand by to handle any unexpected contingencies. If I or any of my defenders die, a solar swoops in with resurrection.