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