Hold up. So who are you targeting? Witch elves/SoS? They won't be disrupted, they don't really fight in formations. Executionrs? They are in the back, frontline won't be affected too much. Rank and file? But they are the easiest for us to handle in melee. Everyone at once? But we don't have the firepower for such an unfocused volley to do meaningful damage.
As for our heroes, they won't have to be everywhere at once. Only at the spots where our troops are giving way, and there should be enough of them for that.
Okay, it appears that I need to make this clear then.
The Flagallants go first, and force their way into the ranks of the Shackled before blowing up, and opening a corridor for the Cavalry to charge through
This runs into the hinted at issue of the Druchiis spear users forming up and forming a shield wall that, when combined with the remaining warbeasts, will possibly stymy the charge or at least force them to maneuver around it before they can hit the less armored Druchii elite infantry, thus minimizing the damage they can do.
However. A shield wall does present a very cohesive, attractive target for bombardment. Which our artillery can take advantage of to inflict damage, along with our ranged defenders to essentially force the backline of the Druchii infantry to space themselves out to avoid presenting an easy target.
Which means that this would let our cavalry bite
very deeply into the Druchii lines, maintaining the full power and momentum of their initial charge, and trampling a shit ton of Druchii infantry before they can form up and make their way back towards the open gates.
All the while artillery and gunfire continue pouring into those Druchii lines, exacerbating the issue.
That's how this would play out with the initial couple of volleys, and it would be incredibly effective. By that point the armored Shackled should be so diminished as to no longer be a factor, and the Druchii would be unable to muster a large enough force to pin our cavalry behind their lines because our artillery will actively punish any such concentrations of force.
It's not about shooting randomly into the lines, it's about all three(four?) elements of our defense compounding to punish the Druchii for forming up like this.
Which is why I'm putting so much confidence in this maneuver. Against a lesser siege, this kind of move would straight up break a siege line quite ruinously.