As for a specific one liner for the situation provided, it'd probably be quoting Vegeta, saying "Welcome to the end of your life, and I promise you it's going to hurt."
For everyday one-liners, one time when I was playing MW2 online, the enemy team was about to take a part of the map to flank us. I ran in with my Spas, going "One does not simply walk into Mordor!". Needless to say, they didn't take that part of the map.
Also when I snipe, I tend to go "Quack!" after every sniper kill. Reliving the good old Duck Hunt days. If I'm playing with a group of friends, I tend to say "Hey(insert name" before taking them out with a headshot.