Halo. Assault on the Control Room. That loooong assed cavern that leads back up to the surface after you've had to go underground and cross that big chasm, usually using a scorpion. This was in co-op, btw.

I take out a grunt and jackal patrolling the cavern with my sniper rifle, then I zoom in on one that is quite a ways up it. I says to Barry Manilow I says, "I bet I can bounce a bullet off his shield and through his skull." and, just as he scoffs at my hubris, I do exactly that.

Arguably my finest hour.