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