Don't wait. Immediately poke him through the chest. It's dark, and more difficult both to see and to deflect a stabbing weapon than a slashing weapon. No need to be fancy. Chest is a big target. Lungs are hard to miss. Put a big hole in one of them. Orient the blade parallel to the ground so as to slide cleanly between ribs with minimal resistance.
After stabbing, pull out and repeat. Don't be an idiot and stab once then leave it there. He won't die immediately and the sword is the only thing between us and the baton. Be aware that with a sword in his chest, there's nothing stopping him from taking swings at us during his frantic, panicked final moments of life, and our sword being in his chest robs us of a parrying tool.
Also, don't get stuck between him and the car.
Stab, yank, stab. Back off. If he screams, off with his head. Cut from the front rather than the side. We probably lack the technique for a clean decapitation, but the front of the neck is easy to cut and the blood filling his throat will make it difficult for him to be very loud.
If possible, try not to get blood on ourselves.
Go on the defensive, but try to get in a hit or two
He's already wounded, and he took a blow to the head. He's probably disoriented and not thinking straight. Now is the best time to attack. We also have a range advantage, and while his weapon is only dangerous if he hits us with it, ours is dangerous if he tries to grab it. There's no reason to give him time to recover enough to do the sensible thing: scream and run, then call for help.