I played online for the first time over the weekend, and I saw a guy using some sort of gun that wiped absolutely everything out. Everything! He even killed the Rakk Hive after about 5 shots!
What on earth was that gun?
He was most likely using a modded gun. I generally play with a group of people I know, but if I ever go on xbox live with randoms, it seems like they always have modded weapons and shields.
The user was most likely using a "hacked weapon". These items have a habit of showing up in public online play.
A hacked weapon is effectivley a player-made item cheated into the game to give the player an unfair advantage. Once you know how to modifier the game to allow your own weapons, it is easy to allow for bonuses like 10000x experience gain, or 999x damage multiplier.
It makes the game a cakewalk, but can often ruin the fun, especially for honest players trying to play alongside the "hacked" player. The best thing you can do is play with people you know, or friend players you know you can trust.