I found an easy spot to accomplish this after you complete the Bloodlands. Go left to the Highlands and proceed until you get to the second checkpoint. The video below starts from the second checkpoint. Die here and then kill the flying enemy. Make sure your sword is ignited with flame or else it won't do enough damage and then jump left over the gap to re-spawn the large monster and several witches.
There will be several easy to kill enemies spawning here (witches, skeletons) and you want to make sure that at least 8 are stacked up right next to you. Perfectly block any attacks you can but do not perform a critical counter attack until you have enough enemies stacked in front of you. You can just hold the block button, but I found that my health dropped too quickly. Getting the perfectly timed block would stun the attackers and prevent me from losing health.
When you have 8+ enemies stacked up, perform a perfect block and then the critical counter attack. If you kill enough with your counter, the achievement will pop. If you didn't get them, just run right off the ledge and try again from the check point to your right.