Full list of all 54 Enter the Gungeon achievements worth 1,250 gamerscore. It takes around 80-100 hours to unlock all of the achievements in the base game.
The base game contains 51 achievements worth 1,000 Gamerscore, and there is 1 DLC pack containing 3 achievements worth 250 Gamerscore.
Clear the Third Chamber 30 Times
Open the shortcut to the Third Chamber
Access the Temple
Win a wager against the Gunsling King 5 times
Clear the Second Chamber 40 Times
Kill a boss after covering it with glitter
Kill 100 enemies by knocking them into pits
Clear the First Chamber 50 Times
Spend big at the Acquisitions Department
Kill a frozen enemy by rolling into it
Open the shortcut to the Second Chamber
Always be flipping. Guns are for flippers
Avenge Manuel
Get the jump on a Mimic
Access the Oubliette
Why
Complete the Tutorial
Collect five Master Rounds in one run
Complete the game with the Sorceress's Enchanted Gun
Clear the Sixth Chamber
Construct the Bullet that can kill the Past
Complete all 4 main character Pasts
Kill the Pilot's Past
Kill the Convict's Past
Kill the Marine's Past
Kill the Hunter's Past
Defeat the Boss of the Fifth Chamber
Clear the Fourth Chamber 20 Times
Clear the Fifth Chamber 10 Times
Carry a large amount of money at once
Map the first Five Chambers for the lost adventurer
Deliver the Golem's replacement arm
Complete Frifle's Challenges
Ace Winchester's game 3 times
Unlock the Bullet
Unlock the Robot
Open the shortcut to the Fourth Chamber
Open the shortcut to the Fifth Chamber
Populate the Breach
Steal 10 things
Fall at the last second
Kill 100 enemies by dropping chandeliers
Kill 100 enemies while riding in a mine cart
Die in the Past
Kill a Jammed Boss
Accessed the Hidden Market
You've met with a terrible fate, haven't you
Complete the game with Beast Mode on
Kill the Robot's Past
Kill the Bullet's Past
Complete Daisuke's trial
Defeat an Advanced Boss
Take Revenge
Complete Tonic's Challenge