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