Dr. Atominus

First thing is that you need to defeat the 1st boss to gain the ability to shoot projectiles. After that return to the library. There is a hidden entry in the lower right corner to another room. You need to shoot the white cover to destroy it and gain access. Inside this room is a medicine tube. I know of a 2nd location as well. When you enter the water zone, move 2 screens to the right. You should be in a room with a man on a screen that will talk to you and a tank of water on the right side. There is another white barrier to destroy in the lower right corner to enter another room with another medicine tube.

Boss: 1:45
Medicine Tube: 3:15
Water Zone: 4:00
Kill 5 Fish: 4:20
Survive Drowning: 4:55

Achievement Walkthrough - Updated to 2000GS: