Future Design Idea: Detect Essence.
Our work in anti-magic charms has proven that magic exists around Arstotzkians in the air. Is it not a logical conclusion that wizards, if not everyone, have a magical "signature"? By applying what we've learned about "ambient magic" in our work thus far, we can create a spell allowing our mages to "sense" magical presences both in and out of their line of sights. This can be used to find Moskurgians hiding like cowards or those too far away to see regularly. The spell isn't "sight", but rather just sensing - the casting mage senses others.
This can be used to spot for artillery, to detect Moskurgian ambushes in skirmishes, and more!
Furthermore, as our anti-magic charms absorb local magic as they should, forces covered by anti-magic charms won't be seen under this spell and thus confusing forces with enemies is a rare occurrence. Of course, there are numerous Arstotzkian soldiers not under the effects of anti-magic charms in the battlefield. Which is why casting mages tend to focus on finding the exact locations of enemies where only their rough location is known. If infantry is clashing in a battle, Detect Essence is rarely used. However, in scenarios such as eliminating enemy ballistae and ranged troops, Detect Essence can be a vital tool.
If it's very expensive, users of Detect Essence will be accompanied by couriers and messengers that run the mage's instructions to nearby artillery emplacements. If it's just Expensive, then the mages of every cannon are trained to use it.
Another possible solution for the sight issue if we ever get around to upgrading our cannons' sights further.
@Andrea: The steam tank could work, though I question the use of wooden armor when the enemy is fielding extremely strong ballistae. Why bother fielding a tank if the enemies' weapons will just ignore its armor? We should rather use iron armor if we visit that idea. And regarding expense, we really just need to standardize production, as the problem with the HCx-E variants is that their barrels must be specially crafted for each one. We don't have to resort to dispel-able crystals to get it working.