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