Depends on the level of light then, and the position Of the enemy. I'll admit that low light shouldn't be that hard, but i get the impression The battle takes place in what is effectively total darkness, with low light only around the entrance. Assuming that the grimms eyes aren't that vibrant, you're still just swinging and aiming at the general directions of where movement happened. And if you want to hit anything, you don't aim where they were, but where they're going to be.

Of course if the Grimm are moving in predictable patterns even total darkness won't help them. It'll just be a matter of warding off their attacks long enough for them to run into a weapon.