Okay but think about it like this. At some point they will find a way to convert gaseous investiture (Light) into solid (Metal). That would pack a lot more punch. Since anti-investiture doesn't actually interact destructively with matter, I was thinking this for a device:
Two large sheets of metal investiture and metal anti-investiture separated by a highly pressurised gas/liquid so that they do not touch. The sheets should be as thin as possible to maximise surface area of contact, because upon contact anything not too close by would be knocked away by the explosion and would not actually get a chance to annihilate. When ready for boom, depressurise chamber, metals touch, kapow. Make these sheets several hundreds of metres wide and you're basically shooting a giant frisbee of doom at whichever unfortunate civilisation has insulted your horse.
Edit: Actually, you could easily layer this several times over so it wouldn't need to be so long. You could make into any giant <insert your favourite 3D shape> of doom.