Here is the full list of all 27 Titan Souls achievements.
Beat the game in under 20 minutes
Kill the Knight after destroying all pillars
Trigger the Guardian's switches with the wrong hands and kill it
Produce 50 coins in the Chest fight
Remove all teeth from the Mountain Titan then kill it
Destroy all icebergs
Kill the Sludge titan with no more possible divisions
Kill the Brain with a flaming arrow
Slay all Titans in Iron Mode
Beat the game in Iron Mode
Beat the game in Iron Mode with New Game+
Drop a bomb on the Lava Blob Titan's head
Beat the game without rolling rolling rolling (in No Roll mode)
Kill the Broken Statue Titan while pulling back the arrow
Beat Hard Mode
Kill the Skull Titan as it spins
Kill the Rolling Titan in under 5 seconds
Finish in under 10 seconds...
Kill the Eye Cube while pulling back the arrow
Beat the Game
???
Knock yourself out in the battle for the Titan Soul
Cut all the vines off the Plant Titan
Succumb to the effect of spores for 30 seconds total
Open the second Sealed Gate
Open the first Sealed Gate
Slay a Titan