To get the following weapons you must kill the indicated enemy and then collect the weapon when they drop it.

Unlock Redeemer (Pistol):
Kill Gunslinger.

Unlock Sniper Rifle:
Kill Skeleton Sniper.

Unlock Four-Barreled Shotgun:
In Hangtown kill Fat Cowboy Zombie.

Press R3 to activate your 'blood' vision which work like a thermal scope highlighting enemies in a bright glow making it easier to shoot them from a distance.

Unlock Enemies 1:
With 65% accuracy complete Bonyard.

Unlock Enemies 2:
With 20 or more Melee kills complete Showdown.

Unlock Showdown Animatic:
On Shootist difficulty complete The Right Train.

Unlock Wrong Train Animatic:
With 65% accuracy complete The Wrong Train.

Unlock Cowboy Animatic:
With 30 or more Melee kills complete Torture Maze.

Unlock Weapons:
With 70% accuracy complete Morning After.

Unlock Environments 1:
Without any deaths complete Darkwatch Outpost.

Unlock Environments 2:
On Deadey difficulty complete Invasion

Unlock Environments 3:
In 10 minutes or less complete Curse of the West.

Unlock Attract Mode Movie:
With 80 or more kills complete Dead Light Prism.

Unlock Highmoon Trailer:
In 15 minutes or less complete Rescue.

Unlock Rescue Intro Movie:
With 20 or more Headshots complete Hangtown.

Unlock Darkwatch Archives:
On Deadeye complete all Gunslinger levels.

Unlock Bosses Art:
On Shootist difficulty complete Curse of the West.

Unlock Vehicles Allies Art:
With 20 or more Headshots complete Deadfall.

Unlock Evolution of Jericho Concept Art:
On any difficulty complete the Single Player campaign.