What's the best way to clean the glue of protective film which is coated on the elevator door? And the fastest way?

nan

You can try some essential balm.

Wiping the door using water with washing powder at first and scrubing it hard with plastic brush. That's the only way I know.

I do not suggest to use water (water with washing powder etc.). You can use oil. If possible, you can buy some special stainless steel cleaning oil. If the door is painted, machine oil is okay.