Here is the full list of all 15 Skull's Impossible Quest achievements.
Beat Level 1
Beat Level 2
Beat Level 3
Beat Level 4
Beat Level 5
Beat Level 6
Beat Level 7
Beat Level 8
Beat Level 9
Beat Level 10
Beat Level 11
Beat Level 12
Beat every level with no upgrades enabled
Collect every upgrade coin.
Beat every level with upgrades allocated!