To V4N1Sh, got it, but even if it don't proc with Q, It procs when you exit stealth, so i presume it works when you hide on a bush and then storm with your Q as soon as you see someone? Correct me if i'm wrong, but i got your tip!
Edit: yes you can go into bush and it will proc, but it's as good as nothing (-_-), i will edit the guide, Sudden Impact isn't good anymore.

Hi, garen main here, just wanted to let you know that Sudden Impact Does NOT work on Garen's Q as it is only an MS increase only (also tested in sim)