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