It's the first, biggest Zurg, on the left (and closest Zurg to the ride vehicle) and like mentioned above, it's a small hole on his chest between his armor plates. He moves side to side, so it's hard to see and hit.
As for aiming, if I'm having a hard time, I point it away from the targets (everyone else is already shooting at) and walk it up a wall. That way I can see if the laser is off from where I "think" I'm pointing it at. Sometimes it's difficult when everyone else is shooting at the same target and you can't tell if you are hitting it. Pretty sure when you hit it, the gun will vibrate and light up...but I could be wrong about that.
I've also heard if you hit every target in the ride, you get a ton of points, but I've noticed a lot of the targets aren't working lately (especially in the first room). Not sure if that is true.