Since this is new at the time that I am writing this solution I am hiding the semi and full spoilers.
Spoiler Free
This achievement is story driven. It is not missable in a playthrough started on a difficulty. Certain settings in custom and creative disable achievements.
If you have been playing Grounded since early access and still have those saves this achievement is obtainable. The save I completed this achievement on was started in July 2021.
Semi-Spoiler
No major spoilers or walkthrough. Just a vague description of key events and recommendation.
*** Spoiler - click to reveal ***As you complete objectives for BERG.L eventually you will be sent to a specific spot by the shed. Once you complete that objective you will have a nav point leading you to your next destination.
Bring Roly Poly armor and your best ranged and melee weapons. Preferably the armor and weapons are upgraded. Also bring things to light the way and a glider.
Full Spoiler
This is a straight up run-through of what I experienced/did and what I would tell others to do to streamline their playthrough. It's a bit of a read.
*** Spoiler - click to reveal ***To start, if you did not click the Semi-Spoiler I recommend bringing Roly Poly armor and your best ranged and melee weapons. Preferably the armor and weapons are upgraded. Also bring things to light the way and a glider. You can bring healing items, but there are 4 Healbasa (two at the entrance two at the lab before reaching the MANT) in the Undershed literally on shelves in your path on the way to the MANT.
- After you collect the 4 chips for BERG.L memory he will eventually realize he can track Dr. Tully's SCA.B. BERG.L tells you to go to the chair by the shed and gives you an item to repair the SCA.B. More specifically Tully's SCA.B is on the table next to the chair. Either build your way up there or climb the baseball bat onto the chair then jump and glid from the hammer in the chair to the table.
- Repair the SCA.B and listen to the message Tully left. It will cause a nav point to be placed leading you to the entrance to the Undershed.
- Traverse the Undershed to the lab. Obviously, the lab is at the other end.
- The safest/most direct route without building is to begin by following the PVC pipe. Near the end of the PVC pipe in the dark there is a screwdriver allowing you to cross the spider filled gap below.
- There is a Wolf Spider on the other side, but you can sneak past if you don't want to fight. There is also an Ox Bettle near the Wolf Spider, but once again you can sneak by if desired.
- Eventually you see the large hole with two mosquitos, water (filled with Water Fleas) and a web covered cave on the other side.
- You guessed it, you need to get into the web cave. Fastest way down is gliding. There is a parkour via rocks and roots route to the right of the web cave.
- There is a Black Widow and some Black Widowlings in the web cave. You can try running past, but I killed everything. I actually accidently cheesed the Black Widow into the water where it died. Anyway, the web cave is linear so just keep moving forward.
- Once you reach the end of the webbed cave you will see the lab to the right.
- Explore the lab. The normal hit buttons, scan your hand, and interact with computers and/or levers to open locked doors. There is a small amount of backtracking.
- When you finally come across the MANT you will be separated by glass. To get to him you will see a hole in the ceiling. While above the MANT you may think "hey I can just shoot him from up here." NOPE! There is an invisible barrier.
- Drop down and go to town meleeing the MANT until he is dead. I brought my upgraded Club of the Mother Demon. Took only a few minutes to beat the MANT. The MANT has infection fungus meaning he has attacks similar to infected larva and infected ladybugs. These attacks will do the most damage to you.