I like how it gradualy gets harder. The first levels are pretty easy then as you progress it removes the country broders from the map and gives you more and more obscure places.
I found a way to kill a gray faster, but it's hard. attack the gray without touching him so it will harden itself but you won't get a recoil. then hit the gray right after it becomes normal again. if you're timing is right then you get a hit, if it's off you're probably dead.