Nobody Saves the World

Story-related; cannot be missed.

This Legendary Dungeon is in the Northeast, and is technically the fourth dungeon (baesd on level requirements) in the game. Before you can access it, you'll need to complete a side quest to get the security code. You'll need to head east to the demi dungeon where someone is inside with the code. Fight your way up to the end to find the person who gives you the code (you don't actually need to complete this dungeon, but might as well). Head back to the Secret Corporation to use the code and get access (after using another 65 stars to open the door).

Modifiers:

1. Enemies do 1.2x damage (extra 20% damage)
2. Status effects last 5x longer

The first doesn't change much other than how quickly you can die, but the second is extremely important to be aware of. Inside are enemies that will poison you like crazy, and you'll have a very difficult time surviving. Thankfully, there is a workaround. Buy the Poison Dampener perk from the Shop, then fully upgrade it to its fourth level, and it prevents poison from building on you at all. This allows you to completely ignore the big purple blobs that many enemies in this dungeon shoot at you, because the purple puddles literally do nothing with this perk equipped.

As for the dungeon itself, the first floor has you explore three separate linear paths to get three keys. There is a single giant cluster of enemies per path, so be prepared to back up if need be. The second floor is a twisting linear path around the floor and eventually leads into a gated fight. This fight is in a small area with a lot of enemies, so it'll definitely be tougher than previous ones. After getting to the stairs, you'll be able to head up to the boss floor.

This boss fight is far more difficult than the previous three, and in my opinion is the hardest boss in the game. It's a giant version of the crab enemies you've seen. Thankfully, this one doesn't break down into smaller crabs when you drain its health. It only has one ranged attack, and that is shooting purple balls at you. These are very easily dodged. The rest of its attacks are close range attacks, and they deal massive damage. Avoid and/or keep your distance at all costs. The real problem during this fight is that those giant growths spawn, and then continually reset the ward on the boss and any applicable enemies. You should therefore make these huge growths a priority as they'll get really annoying. In fact, this is the first fight where I recommend actually killing all the adds first, because they are finite. The boss has three health bars like usual, and as each is nearing depletion it'll spawn a ton of enemies. Kill them all and then you're free to keep your distance from the boss while you drain his next health bar. Once he's dead, everyone dies and you're finally done.

I ended up using the Slug again for this fight, with Arrow Flurry to remove the Sharp wards and Ravager Rocket to remove the Blunt wards. The Ravager Rocket actually worked amazing against the growths, so I highly recommend it. Keep moving around the arena and keep your distance from the boss, because his melee attacks can take off a huge amount of health. I would immediately try to target the growth, usually firing rockets into the fray to hit it and kill it, then run around the outside of the arena taking out all the enemies, then concentrate on the boss when it was just me and him, using regular Slug attacks. However, i had a tough time with this one and died a bunch, so feel free to use whichever Form you're best with at this point.

This achievement will unlock when you grab the Gem Shard after beating him.