I found a useful strategy for destroying the attack helicopter 
on City of Walkways 2. First, kill the guy on the box with the 
Rocket Launcher, (00 Agent only), with the Meyer TMP, or the 
Wolfram P2K, so he doesn't blow you up while you're doing this. 
Go to the car on the lower level, and pick up the Sentinal and 
some ammo. Then, go over, (on the same level), and stand in the 
spot where you can pick up more Soviet ammo. Aim the Sentinal 
through the gap between the ramp and the upper floor. From here, 
you should be able to see the helicopter when it pauses its m
ovement. Shoot it with the Sentinal during this pause. Eventually, 
the helicopter will blow up, (if you hit it). (NOTE: It's not a 
good idea to use this hint if you're trying to get Wildfire Mode 
in Agent. Usually, the final mission time using this hint is about 
8-10 minutes.)