Here is the full list of all 18 Dungeon of Zolthan achievements.
Defeat Zolthan.
Get the super dash powerup.
Defeat the third boss.
Defeat the third boss without getting hit.
Get the double jump powerup.
Defeat the second boss.
Get the dash powerup.
Defeat the first boss.
Get the higher jump powerup.
Get the rapid fire powerup.
Get all extra life hearts.
Defeat the first boss without getting hit.
Defeat the second boss without getting hit.
Defeat Zolthan without getting hit.
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.