Here is the full list of all 123 Cassette Beasts achievements.
Defeat Captain Codey
Defeat Captain Heather
Defeat Captain Cybil
Complete Viola's quest
Bring Viola to the shipwreck
Unlock Viola
Find a helping hand in a place far away.
Find yourself somewhere far away.
Defeat Captain Gladiola
Defeat Alice
Complete Barkley's quest
Clear 3 Landkeeper offices
Obtain an Ice-type bootleg
Unlock Barkley
Obtain a Fire-type bootleg
Obtain the Pumpkin Vine Ball ability
Get to 5 hearts with Meredith
Defeat Captain Lodestein
Defeat 20 Rogue Fusions
Complete Felix's quest
Meet Kuneko at 4 altars
Meet Kuneko at 3 altars
Meet Kuneko at 2 altars
Meet Kuneko at 1 altar
Meet Kuneko
Help Hoylake with his research
Defeat Captain Skip
Obtain 50 different species
Complete Kayleigh's quest
Try to sneak past a battler by jumping over their head
Meet the Travelling Merchant
Defeat Captain Penny Dreadful
Defeat Captain Clee-O
Obtain the Diveal Swim ability
Obtain a rare sticker
Defeat Captain Zedd
Clear 2 Landkeeper offices
Defeat Poppetox
Complete Meredith's quest
Obtain a Lightning-type bootleg
Defeat 10 Rogue Fusions
Unlock Felix
Obtain an Astral-type bootleg
Meet the one who dutily attends the stations.
Defeat a ghost
Bring Meredith to the mall
Obtain the Electromagnetism ability
Obtain the Bulletino Dash ability
Obtain 20 different species
Unlock Meredith
Start Kayleigh's quest
Defeat 5 Rogue Fusions
Choose a design for Harbourtown's sign
Defeat Captain Wallace
Defeat a Swarm
Investigate a rumour
Defeat a Rogue Fusion
Sign up to the Rangers
Remaster a tape
Obtain a rumour
Clear 1 Landkeeper office
Obtain 10 different species
Unlock Eugene
Complete the tutorial
Obtain the Mothwing Glide ability
Unlock Kayleigh
Obtain Bansheep
Obtain Candevil
Defeat an Unstable Fusion
Defeat an Orb Fusion
Defeat 50 Rogue Fusions
Visit every part of New Wirral
Observe 120 different species
Obtain 100 different species
Obtain 120 different species
Obtain additional data on every species
Obtain a Beast-type bootleg
Obtain a Plastic-type bootleg
Obtain a Plant-type bootleg
Obtain a Water-type bootleg
Obtain a Poison-type bootleg
Obtain an Air-type bootleg
Obtain an Earth-type bootleg
Obtain a Metal-type bootleg
Obtain a Glass-type bootleg
Obtain a Glitter-type bootleg
Obtain the Averevoir Flight ability
Get the Fusion Radar
Obtain a monster form thought impossible to find.
Clear 4 Landkeeper offices
Clear 5 Landkeeper offices
Complete Eugene's quest
Get to 5 hearts with Kayleigh
Get to 5 hearts with Eugene
Get to 5 hearts with Felix
Get to 5 hearts with Barkley
Get to 5 hearts with Viola
Enter a romantic relationship
Solve the riddle of Morgante's song.
Reopen every station
Defy all odds in a place between worlds.
Defeat an ancient force brought back to full strength.
Defeat Lamento Mori
Defeat Heckahedron
Defeat Babelith
Complete Morgante's song
Overcome the essence of fusion.
Come face-to-face with a faceless being.
Defeat Captain Judas
Defeat Captain Buffy
Defeat Ianthe and become a Ranger Captain
Help Sunny start a new life
Complete Frankie's training
Complete Vin's training
Complete 5 noticeboard quests
Complete 20 noticeboard quests
Complete 50 noticeboard quests
Complete 100 noticeboard quests
Open a door long since thought lost.
Win a battle before it starts
Win a battle while ghostly
Win a battle while Gambit is active
Prove your worth to a certain warrior.