Here is the full list of all 25 Brain In My Head achievements.
Kill 5th Boss
Unlock 5th level
Kill 4th Boss
Unlock 4th level
Kill 3rd Boss
Kill 1000 enemies
Unlock 3rd level
Kill 2nd Boss
Start a burner on the second level
Unlock 2nd level
Kill 1st Boss
Kill 100 enemies
Find Meds on the level and use it!
Die once
Kill 10 enemies
Fall into the abyss on any level
Heal once
Make enemies fall on the first level
Unlock 1st Level
Kill 10000 enemies
Die 10 times
Die 100 times
Heal 10 times
Heal 100 times
Heal 1000 times