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