Here is the full list of all 50 20XX achievements.
Clear level 3 with Bankrupt, Famine, Furor, High Tide, Marathon, Purist, Rock, Swarm, Toxin, Undying, and Wrath active.
Clear level 1 with Bankrupt, Famine, Furor, High Tide, Marathon, Purist, Rock, Swarm, Toxin, Undying, and Wrath active.
Complete the game with High Tide, Marathon, Wrath, Undying, and Destiny active.
Complete the game with Marathon, Famine, and Bankrupt active.
With the Undying Skull on, complete the game without picking up any damage upgrades.
Complete the game with Wrath, Famine, Marathon, and Toxin active.
With the High Tide Skull on, complete the game without picking up any maximum health upgrades.
Complete the game with Wrath, Undying, Furor, and Swarm active.
Clear Level 40 in Endless Mode.
Clear Level 25 in Endless Mode.
Clear Level 16 in Endless Mode.
Win 250 Slot Machine prizes.
Complete 10 Daily Challenges.
On Level 8, defeat Shatterbeak without letting him use any interruptible abilities.
Complete a Marathon run within 35 minutes.
On Level 8, defeat 50 Flapps and Eternal Star within two seconds using Force Nova.
Unlock everything in the Soul Shop.
Complete the game having taken damage ten or fewer times.
Complete the game within 28 minutes.
Complete the game with Rock and Purist active.
Complete the game with Final Destination active.
Without Bankrupt active, complete the game without picking up any nuts or nut-granting Augs.
On Level 8, defeat all four Rollsters within a single three-second window.
Have 50 Maximum HP at once (Endless Mode not eligible).
On Level 8, defeat Vile Visage by breaking his horn while he's at or very near full health, then only using basic (including charged) attacks.
On Level 8, survive for 60 seconds without taking damage during The Perforator fight.
Complete the game with at least 3 Prototypes and no System Restore.
On Level 8, defeat Death Lotus with 10 Lotus Spawns alive.
Complete the game with 8 Powers.
Complete the game with 1 HP remaining (and no armor).
Destroy a vending machine -- a victimless crime.
Complete the game with the default primary weapon and no Powers.
Complete a Daily Challenge.
Complete the game with 1 or fewer maximum HP.
Don't touch any surfaces for 10 seconds (Endless Mode not eligible).
Complete the game after entering and achieving glory in all Glory Zones.
Forsake your only true friend.
On Level 8, defeat Kur after reflecting all of his Quint Laser attacks.
Have 250 nuts at once (Endless Mode not eligible).
Equip four non-matching Cores at the same time.
Complete a Rush Job.
Deliver a boss' killing blow with its weakness.
End a run with at least 50 Soul Chips.
Complete the game.
Complete level 8.
On Level 8, defeat both Twin Astrals with falling platforms.
Complete level 4.
Complete a Core Set.
Start a third run in a single game session.
Die 50 times.