The Witcher 3: Wild Hunt - Complete Edition

There is a great place to get this achievement quite early on in the game. It's after you have finished in White Orchard, and spoken to the Emperor of Nilfgaard. You continue your journey to Velen and upon immediately travelling there you end up at Hanged Man's Tree on horseback.

Very near this tree will be a pack of Wild Dogs (5 in total) which will all be level 1, maybe 2. These are very easy to take out with Igni and your sword. You'll want to try and take a couple with Igni first and then a couple of hits with your sword should take out the remaining ones.

I wouldn't recommend trying this until you are at a high level (I did it around 20).

Go back to Velen or any place that has a pack of low level wolves. Remember how they gave you a hard time when you're level 1 or 2 (at least in the highest difficulty they do)? Well, now it's time to get your revenge. With only 1 or 2 swings you can easily take one down, very fast.

Alternative or complementary ways to get them to die quickly is to rely on the incineration effect of the igni sign and then kill the whole lot when at least some have their HP lowered or even using Venon bombs which continuously cause their HP to drop.

If this solution helps you in any way, please leave it a thumbs up to let me know. Thanks!

This might be a little long but its a an easy way to always get this achievement as I have gotten twice while stacking this game both times on Death March difficulty and this always works for me.

Another method of gaining this achievement is to go back to an earlier location in the game and fight drastically lower level foes. Upon having a 5 level advantage or more, a foe's ability to block your strikes drastically falls, eventually failing to block any of your attacks once the level difference is great enough. Having a significant level difference, optimal steel sword for your level in addition to Hangman's Venom, Grindstone bonus, Thunderbolt, Wyvern Decoction and Blizzard potion can really devastate a group of inferior foes. Add in an AoE Aard usage (best used after Blizzard takes effect) and you can quickly stun a large group of foes before slaughtering them all. This is even easier if you have the Piercing Cold mutation, adding a frost effect to your Aard signs with a chance to freeze affected foes or insta-kill those afflicted by both the freeze status and knock-down.

I was able to get this pretty early in White Orchard. At L 3 with Igni and only a slightly better steel sword. While exploring the area, you're sure to find a lot of wolves and some times they are in packs of 5 or more. Try to draw as many as you can and hit them with Igni, then finish off any straggles quickly.