Here is the full list of all 40 Shantae and the Seven Sirens achievements.
Deactivate all the Towers.
Upgrade all magic to max.
Defeated the fifth Siren.
Obtain all magical forms.
Complete a Ghostly Quest.
Unearth a buried treasure.
Learn all magical dances.
Defeated the fourth Siren.
Locate the sunken ship remains.
Win the highest total in a Hula Contest.
Fell for a double cross.
Open a Shock Machine.
Ascend to Armor Town.
Defeated the third Siren.
Enter the Sea Lab.
Defeated the second Siren.
Obtain the Lucky Amulet.
Use the valve.
Uncovered a possible threat.
Reach Tree Town.
Defeated the first Siren.
Have three Monster Cards equipped at once.
Took the tour!
Obtain a Monster Card.
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!
Revitalize cellular functions.
Turn an enemy to an ally by helping them heal.
Witness every magical outcome with Slimes.
Shock a circus squid.
Open all Shock Machines.
Unearthed all secrets.
Avoid startling the Lobster Girl throughout the adventure.
Collect all Monster Cards.
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.