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.