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