Here is the full list of all 16 Soulcaster: Part I & II achievements.
Clear Soulcaster II on Normal difficulty
Purchase all upgrades for Aeox (either game)
Clear Soulcaster I on Normal difficulty
Obtain 5 Soul Orbs
Purchase all upgrades for Shaedu (either game)
Purchase all upgrades for Bloodfire (either game)
Clear Soulcaster I on Hard Difficulty
Clear Soulcaster I without summoning Aeox (Hard Mode)
Clear Soulcaster I without summoning Shaedu (Hard Mode)
Clear Soulcaster I without summoning Bloodfire (Hard Mode)
Clear all Challenge Rooms in Soulcaster I
Clear Soulcaster II on Hard Difficulty
Clear Soulcaster II without summoning Aeox (Hard Mode)
Clear Soulcaster II without summoning Shaedu (Hard Mode)
Clear Soulcaster II without summoning Bloodfire (Hard Mode)
Clear all Challenge Rooms in Soulcaster II