Combine the above.
One faction - or a splinter group within that faction for even more intrigue - has tipped off a human cadre of hunters to attack the meeting. Start the party off with a truck bomb, throw molotovs and other homemade incendiaries after the initial blast and to cover the side/rear exits, then fall back to shooting positions to pick off things that make a run for it. Flee after the first few rounds because the police WILL be rushing there in force, and that alone provides additional problems for the supernaturals.
OR, depending on how much advance warning these hunters have, simply line the building with dynamite. Blow the building up (causing a potentially fun challenge as the collapsing building itself becomes an enemy). Either hit the site afterwards with the "shoot survivors and flee" plan (if they're zealots), or don't even stick around to watch the handiwork (if you want their identities a mystery).
This sounds more and more like a job for a splinter group within a faction. They sic the hunters on the meeting and if successful, this splinter group can take advantage of the power vaccuum left in their organization.
