For this achievement you need to knock over (Scatter) 1,000 Scientists.
This does not
have to be done in one run as that would be impossible, this achievement is cumulative over all your play throughs of the game and all you need to do is fly/land near a scientist to make them scatter (You can also jump by them to help by tapping or clicking the screen)
You can check how many Scientists you have scatter from The Stash - My Profile - Totals (Scroll with your finger or mouse)Note the Nerd Repellant gadget will stop Scientists from showing.
Here's a video showing some Scientists being scattered