Play the "Graveyard" level and shoot the ambulances that approach you. After flames appear from the engine, you have destroyed it. All the ambulances appear early, at the following locations.

In the second area, just keep advancing. When you see the lights, shoot at it and do not remain in its path.

In the fourth area after the trap passage, keep moving forward until you find a fence blocking your path. The ambulance will appear from there. Do not remain in its path.

In the sixth area, go to the right to find a parked ambulance. After some fights, the lights will switch on, indicating that the vehicle will move. Destroy it and do not remain stay in its path.