Here is the full list of all 18 Dungeon of Zolthan achievements.
Defeat Zolthan.
Defeat Zolthan without getting hit.
Get all extra life hearts.
Get the super dash powerup.
Defeat the third boss.
Defeat the third boss without getting hit.
Get the rapid fire powerup.
Get the double jump powerup.
Defeat the second boss.
Defeat the second boss without getting hit.
Get the dash powerup.
Defeat the first boss.
Defeat the first boss without getting hit.
Get the higher jump powerup.
Complete the game with only 3 hearts.
Complete the game with 5 or less deaths.
Complete the game in 40 minutes or less.
Complete the game in 20 minutes or less.