Here is the full list of all 40 Shantae and the Seven Sirens achievements.
Took the tour!
Defeated the first Siren.
Reach Tree Town.
Use the valve.
Defeated the second Siren.
Obtain the Lucky Amulet.
Enter the Sea Lab.
Defeated the third Siren.
Ascend to Armor Town.
Fell for a double cross.
Locate the sunken ship remains.
Defeated the fourth Siren.
Complete a Ghostly Quest.
Defeated the fifth Siren.
Deactivate all the Towers.
Defeated the Empress Siren.
Complete the adventure!
Complete the adventure with 100% completion!
Complete the adventure as quickly as possible!
Complete the adventure with 100% completion as quickly as possible!
Uncovered a possible threat.
Revitalize cellular functions.
Turn an enemy to an ally by helping them heal.
Witness every magical outcome with Slimes.
Shock a circus squid.
Open a Shock Machine.
Open all Shock Machines.
Unearth a buried treasure.
Unearthed all secrets.
Upgrade all magic to max.
Obtain all magical forms.
Learn all magical dances.
Avoid startling the Lobster Girl throughout the adventure.
Obtain a Monster Card.
Have three Monster Cards equipped at once.
Collect all Monster Cards.
Win the highest total in a Hula Contest.
Complete the game without any Health upgrades and all Heart Squids collected.
Complete the game in Beginner or Definitive Mode.
Complete the game in Rule Breaker or Full Deck Mode.