For millennia the Arbol and the Byrond clans have engaged in bloody warfare. A benevolent wizard who is passing by and hears of this decides to stop them once and for all.
He casts a powerful spell that makes the clans invisible to each other. This includes their clothing and any artefacts (British spelling) they are carrying including weapons. It also applies to substances such as dust, powder, solids or liquids that gather or are thrown at them and stick. In other words they cannot be made visible by some trick.
The wizard leaves this non-magical medieval realm believing he has done a great service to all. Unfortunately the clans still want to carry on their vendettas.
How can they fight if they can't see each other? What tactics could they use?
The clans are not invisible to themselves nor to anyone not involved. They are only invisible to the opposing clan members or opposing mercenaries or opposing agents such as trained animals. As stated, no tricks such as throwing powder will make them visible. The wizard has excluded that.
Clarification - if you throw powder and it sticks then it also becomes invisible. The part that doesn't stick will presumable leave a temporary 'hole' but the opponent can then dodge.
They would be given away by brushing against things that aren't attached to them in some way. A branch that got caught in their clothing would become invisible but a branch they brushed against would not.
Trying to join one of the clans in order to settle your own grievances would be treated with great suspicion. You might be a spy. There are very strong bonds of blood in these clans. Marrying into the clan would require a thorough check into your background.