Plan:
Step 1: Consult collected Crimson minds to see if this witch doctor is the leader of the Mastiffs. If the answer is 'no', and I'm guessing that it is, use sharp body parts (spawning new ones if needed) to make the smallest possible incision in the Duffelbag that we can peek out of.
Step 2a: If we are in cab of a vehicle with only one other person, burst free of the bag to slaughter and eat the poor sod.
Step 2b: If we are alone in an open air truckbed, tear ourselves a hole big enough to escape in bird form. Stalk the vehicle in Bird Form. Wait until you have the opportunity to swoop down on a lone Mastiff. Do so, killing and eating him.
Step 2c: If we cannot escape undetected, and there is more than one Mastiff around, take a tiny form to avoid detection, and observe what happens when they arrive at their destination and open the duffelbag.
Step 3ab: If we managed to escape undetected and consume a Mastiff, purchase and consume several gallons of gasoline and an ignition source, so that we have a trump card for what happens next.
Step 4ab: Return to Mastiff HQ the following day. Take the form of the Mastiff we ate, and play dead once more with a new note. 'The Forces Behind the Shopping Center Incident Demands an audience with the leader of the Mastiffs. Not the Cops. Not the Crimson. Not the Which Doctor. Bring this corpse, and one person you wouldn't mind turning into a corpse, to [appointed place] at [appointed time]. This is your last chance to do things the easy way.'
Step 5ab: If the Mastiffs do as you want, give them the offer from my previous suggestion: 40% of the Crimson's Resources for their aid, 100% of the Crimson's resources for an oath of Fealty. If things go wrong, the Mastiffs are dead to us, and we have some prepared fire magic to help fight our way out of whatever corner we are put in.