Sker Ritual

This guide explains how to easily beat all maps and easter egg on nightmare difficulty. You can use it to do hard too.

So the first things I found out were this:

Completing on Nightmare if you haven't already done it on Hard also pops the hard achievements

You only have to do the non-hidden objectives part of the story (Only need to win the first Abraham fight) for the achievement to pop - Just make sure not to die after beating him, instead press and hold to leave the level at the red orb that shows up after defeating Abraham

Video Guide for the story and how to complete it, although I would HIGHLY recommend getting comfortable with this on normal first!

My recommendations for survival is using melee only for points on the first 3-4 rounds. I was able to hold off on buying my first gun until I got to the assault rifle in the weapon upgrade room.

As soon as you can get the health increase perk, but make sure to also be working on an objective. Most escort and hidden object objectives I highly recommend saving a zombie so as to make it far safer.

Upgrade your assault rifle to tier 1 as soon as possible.

I got to the boss fight on wave 19-20ish and all I used was a full ammo Tier 2 assault rifle. So make sure to upgrade your assault rifle twice by the time you enter the boss arena. If only upgrading it once make sure you have money for ammo refills because you will need them.

Perk choices are also important, I mostly went with ones that gave me either defense, reduced enemy damage, or healed me.

Good luck!