You have yet to explain though why I would be drawn to such a densely packed force with terminators backing them up, instead of riding out the barrages, and occupying all the board you left blank. Killing everything else outside of the ruin, and then turn my attention to it. I am not going to leave my sense at home, and attack the thing you want me to attack, not until I decide it time.
Sure your barrage will kill chunks of orks, but everything kills orks. They die in large numbers, even in a sweeping victory. They can stand up to IG leaf blower lists, so one barrage not going to unnerve them.