OVERWHELM

Prior to starting the game make sure you throw on all of the assist options as they do NOT disable achievements. You can find these under Options->Difficulty

Tunnel Worm is the boss located at the Bottom Middle of the map. In this area you will have lower visibility but can use your unlimited ammo to illuminate areas for a brief period of time by shooting into them. Tunnel Worm itself is a relatively simple boss and slow if you have the appropriate handicaps setup, it will tunnel through the map and is vulnerable head to tail for the most part. I believe if you let the hive grow strong enough then the head will develop a hard plate of armor later in the fight making that portion invulnerable to damage but besides that there isn't too much to worry about.

You can see this boss fight in the video below at the 2:29 mark (credit to Indie James)

The Tunnel Worm is located in the bottom middle of the map. You will know you are in the right area when your visibility range starts to get very dark and there is dirt everywhere. After breaking the crystal container, you must fight and defeat the Worm to earn the DARK achievement.

Luckily, you can turn on all the assists in setting which will not interfere with your achievement unlocking so definitely check them all!

Strategy


After awhile of evading and shooting, you'll take down the Worm (signified by it stopping and then bursting into white energy). The achievement will pop after defeating it.

Extra Tips - Order of Bosses to fight
1. Kraken (Bottom right)
2. Lizards (Top right)
3. Worm (Bottom middle)
4. Skullbat (Top left)
5. Bush Spider (Bottom left) - Easiest to fight for it's ultimate form.