If you plan on playing through the game I recommend waiting until later to try complete this as there is an area with enemies that are easy kills.
After you have beaten Stygian, the 3rd boss after meeting Samael, you will gain access to the Iron Canopy area. This area is filled with spiders, but most importantly are the little spiders that tend to hang out on the walls. Note that if you are in need of health that these little guys, if you press
around them, will give you a small amount of health and with the amount of them around in certain rooms it adds up.
For the purpose of this achievement there are multiple areas where you can run towards the walls they are on and get them to follow you (they are usually in groups of 3 or 4). as long as you keep moving they don't really attack you and you can get a big group of them around you and unleash your Blade Geyser to unlock this achievement with ease. At this point in the game I had unlocked Blade Geyser lv. 2. If you are still at lv. 1 it may prove a little more difficult, but is still probably possible, as the room I got it in had about 15 spiders and my attack only didn't kill 3 of them. This, to me, was the first easy point to gain this achievement, as the zombies are the only other likely enemies to do this on earlier, but they are pretty quick and can sneak some attacks in on you making it more difficult.
I hope this helps. Please let me know if you find any issues with this guide. Happy demon slaying
