This achievement is story relevant so you can't miss it.
*** Spoiler - click to reveal ***After getting you hide in the room where the sleeping conventioneer was/will be. Go out of the room, open the second door and you will be shot with the shrink ray. Go through the second door and make it through the mouse hole in the back wall. If you grow before you made it, go back outside the room and get yourself zapped by purple tentacle again.
In the green tentacle room, wait until you grow back, and get the bowling ball. Leave the room and go into the cellar laboratory.
Use the bowling ball on the tentacles guarding the switch.
After the cut-scene you get zapped by purple tentacle again. Talk to purple tentacle, and use the dialog choices 1, 4, 1 and 3. Purple tentacle will then shoot Dr. Fred, defeating himself.