How difficult is it to shoot down a helicopter?

I do apologise for the intrusion... The subject of this posting is not
nice... But I have a good reason to ask for professional advice (and I
am neither a terrorist nor a common criminal):

One of the most notorious criminals of Greece, managed to escape
(again) by using a hijacked helicopter:

http://www.latimes.com/news/nationwo...,4042657.story

This is the second time he escaped using that way. Both times, the
greek policemen did nothing, because they were either corrupted/bribed
or just utter wasters.

My simple question is: How difficult is it to shoot down a helicopter
provided that the greek policemen HAD machine guns and their distance
from the helicopter was around 100 yards?

A similar question to the previous one: What is the most efficient way
to down a helicopter? When I was a kid, I saw a movie with Kirk
Douglas (I think). He was hiding in the mountain forrests with a rifle
and he was being chased by a helicopter. He aimed very carefully at
the tail rotor, the pilot confessed to the copilot that he (Kirk
Douglas) was attempting something very smart, and eventually when he
managed to shoot at the tail, the helicopter was downed.

This is the company that hires the Helicopters:

http://www.interjet.gr/en/index2.asp...eid=helicopter

You can see from the video that the tail rotor is not visible (as it
used to be in older generation helicopters).

Would it be smart enough if the policemen started shooting at the tail
when the helicopter landed, so that none would be harmed and the
prisoners would be unable to escape? I presume that shooting at the
tail propeller could be also efficient but would it bounce many
bullets?

Thanks.