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