Warhammer 40,000: Boltgun

This achievement can be as easy or as hard you'd like it. Middle of solution details the cheese.

From the main menu, choose horde mode and then the highest difficulty.
Note: The difficulty achievements for Horde stack.

There are 12 rounds in an enclosed arena where each round completed will give you access to new weapons as well as keys to open up rooms for new weapons and powerups.

The 12th round is a Boss round.

To start each round, you need to go to the middle of the first floor and pick up the weapon hovering in the middle.

If you want to cheese this and get it done as quick as possible, do the following from the main menu before starting:
Options > Accessibility
Toggle Invulnerability - On
Give all weapons - On
Infinite Ammo - On

This will make it to where you have all the weapons, you can't die, and you have infinite ammo. Doing it this way, it will take around 30 minutes to complete the 12 rounds.

When you complete the 12th round go to the top floor and walk down the newly opened hallway to exit.

This will net you the following other achievements: