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