Darksiders

I found it very easy to get this in the Iron Canopy after clearing the entire dungeon. There is a room where you have to take an object up two elevators to get access to a bomb to destroy a red crystal, and this room always has a lot of the little spiders. When you come from the top of that room and jump down, you should aggro quite a lot of the little spiders and they'll follow you down. Just grab a few more from around the bottom floor and unleash your move. If timed correctly ten of the spiders will go up in a cloud of souls and the achievement is yours.

in Iron Canopy there is a room which has 3 spiders spores (or whatever you like), you know, those big bulbous black thingies, as well as a few trios of spiders on the walls around.

what I did was use the Crossblade to attract the attention of three of the trios (aiming at just one spider of each), those were the spiders on the immedaite left and right on the door as you walk in and the third being either the ones above the door or the spiders that would be the second group on the walls on the right side, then it would be wise to run towards the first large spore to set that off.

In total you should have up to 12 spiders coming at you. let them all catch up by running back towards the door which is where you should strike.

take note that at the time my Blade Geyser was level 2 which is good enough the kill the spiders in one smash without wearing them down beforehand with sword or scythe, also this was done first time going through the level, I don't know the changes after defeating Silithia...

Found a great place for this, its also in the Iron Canopy. You could probably even get the 10 without leveling up the spell, but its good to have it at least Level 2.

There will be a room with 3 chests in alcoves (bottom has souls, middle a Map, top a Beholder Key). You have to clear this room to progress in the game and its not difficult at all.

After getting the Beholder Key from the top chest, return to the open area and grab on the vines, move to the right to get to the door. Use the Beholder Key and progress through.

You will then come upon a room that has a web floor (a huge spider will be hanging on the under part, but don't worry he can't hurt you), Anyway, there will be a movable platfrom that you can jump on. Go ahead and jump onto it, then pull out your crossblade and lock on and attack all the little spiders on all the walls. Don't worry they wont be able to get to you.

Take your time and get them all down, then simply jump off the platform and wait until they are all around you (there is a good 15 spiders at least). Then use the spell, even if its not leveled up a lot you should be able to kill the required 10 easily.

When in the ashlands in the big open area, you use the chrono puzzle to get across, work your way down into the caves till you come across the blue electric bird.

Run around in circles gathering the two groups of them, try and keep them very close to each other.
Then activate you blade geyser.

Saving and reloading before and after if you dont get it the first time helps.

This may also be able to be done just before and while fighting Griever, but this is the area that it worked for me. Im sure there are also other places as well

Found a new room thats even better.
After defeating Silithia(or before it, its just easier to describe the location working backwards).
Leave the Silithia room and go back the way you came,(this is web door number 1). Work your way back and go through webdoor number 2..
Take a left and head down the hole, this next room has a load of spiders on the wall.
Round them all up and use your ability.
Boom, congrats

Another good area to get this achievement later in the game is when you return to The Broken Stair.

The building you enter to get onto the highway will be full of zombies even when other enemies have been replaced by higher level ones. Just run into a corner to get them to all group up on you then unleash Blade Geyser.