Sker Ritual

This guide is a stand-in until I can write out a full text version but there are two videos you can use in the mean-time.

First video teaches you how to do the main story easter egg for the other achievement requiring you to beat Abraham, where the steps come up on-screen.

This second video is in regards to the hardcore easter egg aka the "Hidden objective".

And for a bit of bonus information in regards to this achievement, you have to fight Abraham a second time at the end of the "Hardcore" easter egg. I died during this fight on my first time reaching the fight and a few minutes of sitting at the scoreboard taking a breather and the achievement for completing the hidden objective popped much to my surprise.

I don't know if this is a bug or intentional but I was happy nonetheless.