My Armour is ContemptG 100

To get all the difficulty achievements in this game extremely easily, go to Options > Accessibility and activate all the options below:

* Invulnerability
* Get all weapons
* Infinite Ammo

Now start the game's singleplayer campaign by choosing the Exterminatus difficulty and complete the campaign.

Completing the campaign on this difficulty unlocks the game's 4 difficulty achievements:

* Exterminatus
* Emperor's Mercy
* My Armor is Contempt
* The Emperor Protects

GG