I do like the idea, but that's going to be a multi turn investment, and we need something now.
Bolt of Tubikh Rrahim - A modified form of the Ivory staff used by our wizards. The most complex part of this is empowering it with a wizards focus so that it will still block magic when at great distance from the activator. It is designed to last for half an hour, and effect a similar area to the base spell.
I suggest this as their cannons need magic to work, whereas our ballista need magic to be better then mundane ballista.
Our ballistae can already take out their cannons in a single shot. Unless the range of the anti-magic is considerable, this would not help take out their cannons by that much.
You know what, I think the previously suggested flying machine may not be the worst idea. It would give us a bonus in skirmishes, and leads to spotter-assisted artillery. I still think having the dead weight of a horse is a prohibitively inefficient idea, though. Rather, lets just do
Flying Carpet: Woven from fine cloth on enchanted looms, each thread is blessed and infused with wind magic. Magical runes and circles are worked into the pattern, which needless to say is a beautiful one, designed by the capital's most talented artists. The end result is a thick carpet that, with the appropriate commands, will float above the ground, rise and fall, and move in any direction faster than a speeding horse, whilst magically shielding any passengers from the winds.
These carpets are fairly large, comfortably sitting three passengers- one mage who handles the magical propulsion, and two elite soldiers armed with bows and Firestorm grenades. Alternatively, the soldiers can be replaced by a commander and a mage equipped with a Teletalk rod, allowing for scouting missions, or giving the commander a 'top-down' view of the battlefield, enabling him to give far more effective commands.
Protection of the Higher Truths/Mirage Shield (1) : Happerry
Adamantium (3) : Devastator, Crazyabe, Detoxicated
Flying Carpet (1): NUKE9.13
If, by some miracle, we design Flying Carpets without a hitch, we could use our revision to boost the range of our artillery, which combined with aerial spotters, would beat anything the Arstotzkans could muster.