Re: gun won't work. please help!
vahlirian is correct (although I thought it was the RIGHT mouse button...)
When you hold the mouse button, two bright lines appear on either side of the Reaper as if they were coming down from the ships in orbit.
As you continue to hold the mouse button the lines converge on the target point.
When they join, the fleet (and the Normandy) fire!
Result = Profit!
[EDIT] So you have to dodge the Reaper's fire until you are in a position to stand still and hold the targeting designator (it's not really a "gun") on the weak point long enough. Oh, and you'll have to do it multiple times to finally defeat this one.
Spoiler
I could give you hints on how that that's done, but you might want to try to work it out for yourself first.
