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