Here is the full list of all 48 Flat Kingdom achievements.
Defeat Arachne.
Defeat Plantosaurus.
Activate the first Checkpoint Guard.
Finish your first side quest.
Use your three forms to fight.
Beat a square enemy.
Beat a triangle enemy.
Beat a circle enemy.
Get your first special ability.
Get all the special abilities.
Open the path to the sky tower.
Activate the last Checkpoint Guard.
Defeat Hank.
Defeat Kraken.
Defeat Quetzal.
Defeat Flatepetl.
Defeat the Norns.
Defeat Fenrir.
Defeat Hex.
Defeat the last boss.
Defeat the secret boss.
Defeat an enemy with two or more shapes.
Finish all of the side quests.
Buy your first heart from the store.
Buy all the hearts from the store.
Change your shape 1000 times.
Get 100% coins in Flat Forest.
Get 100% coins in Flatlantis.
Get 100% coins in Flatepetl.
Get 100% coins in Flatsgard.
Get 100% coins in Flatelysium.
Get all the coins.
Get all of the secret scrolls.
Get the final journal page.
Get 1 Game Over.
Get 100 Game Overs.
Earn the normal ending.
Earn the true ending.
Complete the game in Flat Newbie.
Complete the game in Flat Hero.
Complete the game in Flat Legend.
Earn all of the achievements.
Beat 50 Bunny Balls.
Beat 50 Squidclops.
Beat 50 Sparks.
Beat 50 Roundowls.
Beat 500 enemies.
Beat 1000 enemies.