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