Full list of all 54 Enter the Gungeon achievements. It takes around 120-150 hours to unlock all of the achievements in the base game on Windows.
The base game contains 48 achievements, and there are 2 DLC packs containing 6 achievements.
Complete all 4 main character Pasts
Kill the Convict's Past
Collect five Master Rounds in one run
Kill the Pilot's Past
Clear the Fifth Chamber 10 Times
Kill the Marine's Past
Map the first Five Chambers for the lost adventurer
Kill a Jammed Boss
Die in the Past
Carry a large amount of money at once
Kill the Hunter's Past
Construct the Bullet that can kill the Past
Fall at the last second
Defeat the Boss of the Fifth Chamber
Clear the Third Chamber 30 Times
Win a wager against the Gunsling King 5 times
Access the Temple
Access the Oubliette
Clear the Second Chamber 40 Times
Get the jump on a Mimic
Kill 100 enemies by knocking them into pits
Spend big at the Acquisitions Department
Clear the First Chamber 50 Times
Why
Always be flipping. Guns are for flippers
Kill a boss after covering it with glitter
Complete the Tutorial
Complete the game with the Sorceress's Enchanted Gun
Clear the Sixth Chamber
Clear the Fourth Chamber 20 Times
Deliver the Golem's replacement arm
Complete Frifle's Challenges
Ace Winchester's game 3 times
Unlock the Bullet
Unlock the Robot
Open the shortcut to the Second Chamber
Open the shortcut to the Third Chamber
Open the shortcut to the Fourth Chamber
Open the shortcut to the Fifth Chamber
Avenge Manuel
Populate the Breach
Kill a frozen enemy by rolling into it
Steal 10 things
Kill 100 enemies by dropping chandeliers
Kill 100 enemies while riding in a mine cart
Accessed the Hidden Market
You've met with a terrible fate, haven't you
Complete the game with Beast Mode on
Kill the Robot's Past
Kill the Bullet's Past
Complete Daisuke's trial
Defeat an Advanced Boss
Take Revenge
Complete Tonic's Challenge