Select a mission in Moldy Tundra to get started. Mission type and difficulty settings don't matter, so pick what you want and make it as hard as you want it to be.

Look out for a big silo around the outskirts of the map



Once here, go to the back right side of it and head underneath this large red mushroom


When you get close to it, you should see that the flora on the ground is covering a pit. Drop down, hit the button down below, and it'll start loading a new mission called ???

From this point on, you'll have two revives (so three lives in total). Try not to waste them on the way to the boss.

Once the lift you're in opens up, you should see a very long meadow with a big metal sphere in the back. Make your way all the way there. There will be some pits, which you can climb out of, and a couple of enemy spawns that you can ignore if you wish.

Now that you've reached the big structure, it's time to climb it. You'll be starting from the bottom metal beams sticking out with caution tape patterns adorning the ends. You only need to jump once, and you'll mantle up each step you press up against. No need to overcorrect.

Once you clear these steps, turn to the left and you'll see a platform above held up by a few boxes. Climb up from there until you hit a girder on the upper platform

Climb up the girder and run to the end of it. Turn left and drop on the pipe that the girder's laying on top of, and run up to the next platform.

When you get onto the platform, turn right and you should see a spiraling pipe. Go there and run up it until you reach the top.

Upon reaching the top of that, there'll be another pipe right in front of you. Just hop on it and run up to the top. Look down to the left, and you'll see a pit full of engrams. Jump down there. Watch your step.

Time to fight the boss. It's not too bad on lower difficulties. Just shoot it until it dies.

Not visualized, but worth saying:

-Enemies will spawn during the fight, though it should be mostly grunts and brutes
-Cranius will have four weapons attached at all times, which can be either blob grenade launchers or machine guns. The blobs aren't too bad as long as you stay mobile. If you see a machine gun, take it out ASAP or hope it doesn't hit you when sprinting for health regen.
-At the halfway point on its health bar, Cranius will cover itself in a spherical shield and rain down blobs for a bit. It's only temporary. You can get back to the fight right after.

Cranius drops a bunch of loot once a week upon death. Like a lot. I think I picked up 8 or 9 things for my first kill. The weekly reset should be at the same time as the Overkill Assignment reset, so make sure you give ol' Cranius a visit every week before reset.

Killing it also grants an achievement, for those of you that are into that sort of thing.

That's pretty much it here. Apologies for slipups here, as this is my first guide on Steam ever. I'm open to feedback, though.