it's hard, but thats how its done. You need careful hands and precision exacto-knifing to slice the doors out. Dont want to scratch the body up do you?
Otherwise you'd have trouble with putty!
Patience, you go over the door panels lightly with the knife, and keep going around while adding pressure each time. It should come off sooner or later, but let me warn you! Its EXTREMELY frustrating to hold the body and not damage it while still being able to add that extra pressure to the knife!