A "dirty" solution for Day 1: Line your troops up, swords out, and have them march in a row while chopping and swinging wildly at fresh air and being very careful not to hit anything they can see.
Something a bit more refined for future engagements?
- Carefully trained, silent Bloodhounds, who can sniff out your invisible foes while remaining unseen by them.
- Small bells in a sticky/adhesive goop - thrown at your opponent, they turn invisible - but not inaudible!
- Train up assassins to pick out targets and avoid obstacles by sound. That wizard has provided you with a surplus of invisible ninja for your noble cause!
(Something that was not elaborated on - does the wizard tell the Clans? Or are they just pitched up, ready for battle, then wake the next morning to find their enemy is gone, but their tents remain. Scouts sneak in to investigate, then report back that they can still hear the enemy, but can't see them! How long until they realise that they are just as invisible to the enemy as the enemy are to them...)