Here is the full list of all 51 GunDeck[100] achievements.
Defeat the first boss.
Defeat the second boss
Defeat the third boss
Defeat the fourth boss
Defeat the fifth boss
Defeat the sixth boss
Defeat the seventh boss
Defeat the eighth boss
Defeat the ninth boss
Defeat the final boss
Defeat the final boss as Clutch
Defeat the final boss as Healthy
Defeat the final boss as Speedy
Defeat the final boss as Shieldy
Defeat the final boss as Jazz Hands
Defeat the final boss as Monogun
Defeat the final boss as Lucky
Defeat the final boss as Knifey
Defeat the final boss as Stealthy
Defeat the final boss as Introvert
Defeat the final boss as Swappy
Defeat the final boss as Laser
Defeat the final boss as Greedy
Defeat the final boss as Prescient
Defeat the final boss as DNA
Defeat the final boss as Momentum
Do 10,000 POISON damage in a single run
Do 10,000 BLIGHT damage in a single run
Do 10,000 HOMING damage in a single run
Do 10,000 ICE damage in a single run
Do 10,000 MAGIC damage in a single run
Have 500 crystals onscreen at once
Have 750 crystals onscreen at once
Collect 15 pets in a single run
Collect 30 pets in a single run
Reach Level 50 in Infinity
Reach Level 100 in Infinity
Kill ANYTHING with a backwards gun
Hold one gun of each element
Pick up 5 Y-shaped guns
Defeat the first boss on B side.
Defeat the second boss on B side.
Defeat the third boss on B side.
Defeat the fourth boss on B side.
Defeat the fifth boss on B side.
Defeat the sixth boss on B side.
Defeat the seventh boss on B side.
Defeat the eighth boss on B side.
Defeat the nineth boss on B side.
Defeat the final boss on B side.
Defeat the final boss on B side as Clutch Plus.