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