Nobody Saves the World

Story-related; cannot be missed.

This Legendary Dungeon is located in the northwest corner of the map, and is technically the fifth dungeon (based on level requirements) in the game. Nothing is needed to access this dungeon aside from the 65 stars to open the actual door.

Modifiers:

1. Break wards to get 300 Mana
2. Signature hits (i.e.  attacks) don't regen Mana

As you can tell from these modifiers, if you're reliant on special attacks that use mana, you're going to have a tough time with this dungeon. The wards are Light and Dark, so you should absolutely use a Form whose Signature Attack (i.e. ) is either Light or Dark so you don't have to rely on Mana to break one of the two wards. For the other, use something that doesn't cost much Mana and recharges quickly. I personally used the Slug, whose Signature Attack is Light, and used his Slime Slide to deal Dark damage, because Slide Slide uses almost no Mana and doesn't have a recharge delay.

The first floor of the dungeon requires you to kill 142 enemies to open a gate. Keep your distance since they will be present in heavy amounts. As soon as you've killed enough, double back to the gate to go up the stairs. The second floor is a winding linear path that starts with a gated fight that is surprisingly easy. Keep on the move, and there aren't very many enemies that spawn. After the gated fight, there are some traps and many more enemies, but you can use the traps to your advantage to kill enemies quicker. Take the steps at the end up to the boss fight.

The boss is a huge version of those four-legged creatures that throw boomerangs at you, which you've encountered many times in this dungeon. Surprisingly it doesn't get any wards though, just adds tons of enemies with wards. As a result, I recommend just unloading on the boss nonstop to finish this fight. There aren't any phases or different attacks as you progress, it's literally just a large version of those pretty basic enemies. I ended up using tons of Familiars and they absolutely destroyed this boss in seconds, so consider doing the same (I used the Magician's ability).

This achievement will unlock after picking up the Gem Shard.