Here is the full list of all 22 MidBoss achievements.
Meet the cat merchant
Possess your first creature
Destroy 50 inanimate objects with magic missile
Liberate 450 cratefish
Gain a major blessing from all four gods
Cheat death
Share a death card on social media
Import a death card
Start a game with 6 pieces of grave goods
Beat the game
Get a lucky drop with a x16 multiplier
Unlock all retro mode filters
Kill the skeleton and zombie from the intro cutscene
Master every form in the game in a single run
Accumulate 5,000 yarnballs in a single run
Pretend to be a smithy 200 times
Kill 6 creatures with a single attack
Vandalize 50 shrines
Obtain the golden ticket
Break the rules of engagement and beat the lich lords
Go through the glitch door
Unlock the power of Mid OS and boot up MidBoss Classic