Here is the full list of all 15 Ultionus: A Tale of Petty Revenge achievements.
Clear Zone 0 without taking any damage
Complete Zone 1 without getting killed by a frog even once.
Kill 10 large water serpents without losing a life.
Kill 4 orange centipedes without losing a life
Watch the entire credits without skipping.
Complete the game without buying anything from the shops.
Play Zone 3 all the way to the boss without letting a single enemy escape.
Trigger 7 mines via proximity without losing a life.
In Zone 4 ride a platform into lava.
Buy all weapon upgrades.
Buy all armor upgrades.
Complete the game with a bad ending on any difficulty.
Complete the game on Easy difficulty with a good ending.
Complete the game on Normal difficulty with a good ending.
Complete the game on Hardcore difficulty with a good ending.