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