Go to the regular siva strikes. Run at least 10 you will average 1 key per 7-8 strikes this way. Then leave siva strikes and go to the the will of crota strike and do the 320 lvl strike open the chest at the end and hope you get a grasp and not a bond. It may be a crap roll grasp. It from what it looks like you just want one as of now so this is the way to get it. Now, I have killed omnigul at the bottom of the hill and had a skeleton key drop but I was in the siva strikes when it happened. If you don't get a grasp the first time you open the chest. Rinse and repeat starting with the siva strikes again to get another key. Hope this helps and good luck.