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