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