Okay, gotta handle this one round at a time. Transforming is the quickest and most reliable method of healing Viserys of a significant amount of damage. True Dragon form will bring him up to 117 HP. We need to get him out of the danger zone, because if he goes down, we're done for.
DP has confirmed that the Fire Giants are firing from the hallway, so Varys can temporarily plug the entrance with a Brilliant Barrier as an Immediate Action, before anything else hits us. The Force barrier will only have 70 HP, but the 20 Hardness will mean it can absorb a bit of punishment, certainly long enough for Viserys' Resilient Sphere to block it in a more thorough manner.
The Rune Giant is on its last legs and not quite in range of us for its Death Throes explosion, but I'm not willing to completely chance that. Garin finishing it off with Darkbolts will be followed by another Brilliant Barrier to block the explosion. The spell is an Immediate Action, so it will come into being before the explosion.
[X] Giant Wrangling
-[X] Viserys uses Wild Arcana to cast a Brilliant Barrier (Hardness 20, 220 HP Wall of Force) as an Immediate Action to block the entrance into the chamber, then assumes True Dragon form as a Standard Action, automatically healing 18 HP and gaining 90 additional HP from his enhanced Constitution.
--[X] If necessary to prevent the Rune Giant from closing to range before it dies, he uses Amazing Initiative and his Belt of Battle to gain another action in order to use a Wall of Scales or Brilliant Barrier spell via Wild Arcana or Resilient Sphere to impede it and/or block the explosion, as needed.
-[X] Varys also uses a Brilliant Barrier charm (Hardness 20, 70 HP Wall of Force) to block or impede the Rune Giant, if necessary, as well as helping block the explosion from its death. If that isn't necessary, she instead uses it to reinforce Viserys' barrier. She flies to Richard and begins applying doses of Restorative Ointment (heals 1d8+5 HP per dose).
-[X] Garin targets the Rune Giant with all seven bolts from a Darkbolt (2d8 damage per bolt, DC 21 Will save to avoid being Dazed for one round) spell. He then uses a Brilliant Barrier charm (Hardness 20, 70 HP Wall of Force) as an Immediate Action to block the explosion from the Rune Giant's death.