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.
Clear the Sixth Chamber
Open the shortcut to the Fourth Chamber
Always be flipping. Guns are for flippers
Kill 100 enemies while riding in a mine cart
Clear the Fifth Chamber 10 Times
Unlock the Bullet
Complete all 4 main character Pasts
Kill the Pilot's Past
Die in the Past
Carry a large amount of money at once
Kill the Convict's Past
Clear the Fourth Chamber 20 Times
Kill the Marine's Past
Fall at the last second
Unlock the Robot
Open the shortcut to the Third Chamber
Deliver the Golem's replacement arm
Clear the Third Chamber 30 Times
Map the first Five Chambers for the lost adventurer
Kill the Hunter's Past
Construct the Bullet that can kill the Past
Defeat the Boss of the Fifth Chamber
Clear the Second Chamber 40 Times
Win a wager against the Gunsling King 5 times
Accessed the Hidden Market
Clear the First Chamber 50 Times
Kill a boss after covering it with glitter
Kill 100 enemies by knocking them into pits
Why
Kill a frozen enemy by rolling into it
Spend big at the Acquisitions Department
Open the shortcut to the Second Chamber
Access the Temple
Access the Oubliette
Get the jump on a Mimic
Complete the Tutorial
Collect five Master Rounds in one run
Complete the game with the Sorceress's Enchanted Gun
Complete Frifle's Challenges
Ace Winchester's game 3 times
Open the shortcut to the Fifth Chamber
Avenge Manuel
Populate the Breach
Steal 10 things
Kill 100 enemies by dropping chandeliers
Kill a Jammed Boss
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