There are a lot of guides around the web, like the VG24/7 or the gosunoob one.

The easiest and fastest way to get the achievement, all done in White Orchard.

I provided the fastest path in the image below, starting east of the Abandoned Village. There are 6 in places of power in White Orchard, we need only the 5 different ones. It consists in having all the buffs from places of power active at the same time. This is rather easy, and can be done from the beginning in White Orchard. I did it in this very order, but you can mix it up as best suits you, just do it all in less than 4 seconds.

You “simply” have to counter-attack, attack, throw a bomb and use a sign. I don’t know why this achievement is hidden, I only discovered it while close to the end of the game. What Was That?, Power Overwhelming, Card Collector, Munchkin, Overkill, Humpty Dumpty and Master Marksman achievements. I decided to toss out a few tips that could speed up the process. I’ve played the entire campaign on Death March! and done most of the achievements, except a few ones secret or grindy (listed below) that I completed after finishing the game.

Witcher 3 overkill achivement