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