See, I didn't know anything about the Krakstorm, probably because of my newness. Also, can any unit pilot a weapons platform? If so, that makes D-Cannons a lot more survivable. Honestly, I probably still wouldn't have called Ra going there, even if I knew these things. As for the Dragons, you don't have a transport and would probably not want them footslogging. I'd bet you have a unit for each emplacement. One for the Fortress and one for the Bastion and they pile out if a vehicle needs killing. Unless that is not allowed because Ra is already in the Fortress, in which case, they may use the fortress for cover or footslog.
Ah my bad, you can find the rules for the fortress of redemption and the imperial bastion in the main rulebook as well as rules for using them.
The fortress itself can hold 4 squads so yes the fire dragons go in the bastion and the fortress. The exarch uses the Icarus Lascannon in each section (which is just a las cannon with a 96' range. They exist to kill enemy flyers (along with the broadsides). However they can also kill any other vehicles and along with Tank Hunters for re-rolls and Crackshot to deny cover saves they do a great job.
Also because the Exarch is a character if i roll a 6 to hit i can choose where the wound goes allowing me to snipe models in squads.
They also provide poit defense incase something big deepstrikes next to me, the heavy flamer is for when they are charged, and the dragons in the bastion also use the 4 heavy bolters allowig them to inflict damage up to 36 away.
Can you tell me where the d-cannons go and why, also would they be joined by any character?