Here is the full list of all 18 Spidersaurs achievements.
Complete Episode 1
Complete the Tutorial
Complete a Stage without dying
Defeat a Boss without getting hit
Find a Mini Meat
Stack a Weapon
Complete Episode 2
Complete Episode 3
Complete Episode 4
Complete Episode 5
Complete the Game
Achieve the True Ending
Complete Arcade Mode
Defeat a Boss with the Lv1 Pea Shooter
Use a single weapon for one entire stage
Complete a stage without getting hit
Complete the game without dying
Complete a stage without standing still