This is a way to have a full scale "war" with your local crew. It'll also work for conventions and could easily be applied to OCTGN as well I imagine. Note the set up is simple to allow for flexibility and customization for specific needs.
Step 1: Pick sides. Easiest is good vs evil with neutral being allowed as well. Everyone picks a mage and one mage only for the war. The canon story stuff lists the "good mages" as the Priestess, Priest, Paladin, Straywood Beast Master, and Wizard. Evil constitutes both Warlocks, Necromancer, Bloodwave Warlord, and Siren. Yeah I'm over simplifying things but whatever this is an example. Rest of the mages go I to the Neutral pile, they can fight for whatever side.
Step 2: Pick stakes. In my town we're fighting for our "official" nickname among the MW community. But maybe at a con or something you could give out small prizes. Stakes are important though as it gives you something to fight over later.
Step 3: Fight. This isn't a tournament, it's a campaign. Players fight matches and if you lose your mage is dead. You're out. First side to eradicate the enemy wins. This could be wrapped up in one "battle" or event or it could be prolonged.
Again simple, flexible, and engaging.