Here is the full list of all 62 Sullen achievements.
Select Practice mode on the main menu.
Beat the first stage in Vexed.
Complete the tutorial.
Die for the first time.
Beat Vexed on Normal mode.
Beat Vexed on Normal mode without using checkpoints.
Beat Angry on Normal mode.
Beat Angry on Normal mode without using checkpoints.
Beat Furious on Normal mode.
Beat Furious on Normal mode without using checkpoints.
Beat Livid on Normal mode.
Beat Livid on Normal mode without using checkpoints.
Beat Wrathful on Normal mode.
Beat Wrathful on Normal mode without using checkpoints.
Beat all of the levels on Normal mode.
Beat all of the levels on Normal mode without using checkpoints.
Earn all other achievements.
Beat the second stage in Vexed.
Beat the third stage in Vexed.
Beat the fourth stage in Vexed.
Beat the fifth stage in Vexed.
Beat the sixth stage in Vexed.
Beat the seventh stage in Vexed.
Beat the eighth stage in Vexed.
Beat the first stage in Angry.
Beat the second stage in Angry.
Beat the third stage in Angry.
Beat the fourth stage in Angry.
Beat the fifth stage in Angry.
Beat the sixth stage in Angry.
Beat the seventh stage in Angry.
Beat the eighth stage in Angry.
Beat the first stage in Furious.
Beat the second stage in Furious.
Beat the third stage in Furious.
Beat the fourth stage in Furious.
Beat the fifth stage in Furious.
Beat the sixth stage in Furious.
Beat the seventh stage in Furious.
Beat the eighth stage in Furious.
Beat the first stage in Livid.
Beat the second stage in Livid.
Beat the third stage in Livid.
Beat the fourth stage in Livid.
Beat the fifth stage in Livid.
Beat the sixth stage in Livid.
Beat the seventh stage in Livid.
Beat the eighth in Livid.
Beat the first stage in Wrathful.
Beat the second stage in Wrathful.
Beat the third stage in Wrathful.
Beat the fourth stage in Wrathful.
Beat the fifth stage in Wrathful.
Beat the sixth stage in Wrathful.
Beat the seventh stage in Wrathful.
Beat the eighth stage in Wrathful.
Complete the tutorial with all 3 lives remaining.
Use a checkpoint.
Use two checkpoints in one play.
Use three checkpoints in one play.
Use four checkpoints in one play.
Complete a stage in Practice mode.