Yeah 50 is the minimum needed at that part. However, you need two spells to kill them all. The cloud only kills like 3 I think, leaving some alive. You need to follow up with another spell, but I think even then, 1 lives and gets away. That's assuming you don't have a magic using extra class like druid. I used the clouds to kill some then followed up with a druid spell which used grass to kill the rest of them no problem. No survivors. When I didn't have a druid class, and used 2 regular spells, like cloud and another spell. I think one got away. If you have enough blades, 50 minimum too I think, you can kill them all as well, been awhile since I used blades on them.