The Gunstringer

There are eleven special conditions within the DLC that act as difficulty modifiers. It's more or less random as to which condition you actually get, so it'll take a few playthroughs to get them all. The conditions are:

Normal - Nothing is changed.
Shotgun Showdown - You get a shotgun!
Flamethrower Frenzy - You get a flamethrower!
Turbo - Enemies move faster.
Metal Dudes - All enemies take two shots to kill.
Single Shots - You only get to target one enemy at a time, there are far fewer enemies.
Tiny Gun - Smaller reticle.
Lefty Flip - Firing with your opposite hand.
Backwards Flip - The most irritating special. Aiming right and left are reversed, but up and down are the same.
Dual Pistols - You've got a gun in each hand!
Auto Guns - Just like the rapid shooting in the standard game.