Shadows of the Damned

To earn this achievement, firstly you will have to defeat a boss. Here's how;

Stick to your machine gun or your Boner. This boss will have a few attacks right out of the gate.

~Attack No. 1: He'll fire large glowing feathers into the bridge. These will pollute the air with darkness, so you need to light shot each feather in order to defuse the darkness.

~Attack No. 2: He'll hover next to the bridge, rear back his head, and fire his tongue out at you. Just dodge roll away from this.

~Attack No. 3: Not really an attack. But he'll unleash some small flying insects that will hover around the spotlight on the bridge. Use the Johnson Bash to get rid of them quickly.

The first thing you should notice is the spotlight on the bridge. All this really does is point to the Boss' location while you're fighting it. It does come in handy though, as this guy is all over the place sometimes. As for damaging him, just blast away at that big red core in its stomach.

After you destroy the core, he'll fly off and reappear with glowing red shoulder sections on the top of his wings. He also gains a new attack;

~Attack No. 4: He'll fly off and grab a huge boulder. He'll hurtle the boulder at the bridge and you'll go tumbling over the edge, a QTE comes into play to pull yourself back up. Just shoot the boulder before he can launch it at you - sorted.

After you've destroyed the orbs on the wings, he'll jump onto the bridge and start walking towards you. Light shot the glowing area by his crotch which will reveal another red orb.
All you have to do here is plug away at it and keep your distance. When he jumps over top of you dash the other way, turn around, and keep firing.....rinse and repeat until the boss is killed.

Now that the boss is dead, you have to walk toward the gem. After that, you will have to do a short run through the darkness, collecting drinks along the way and top up health if required.

Reach the end of the darkness and....*pop*!
Achievement unlocks.