In reply to Zmerr, whilst his skin is resistant, his insides are not. If one were to break a bone, or cause internal bleeding, it could possible be fatal, depending on the severity (e.g. a broken arm won't be very fatal). Also, a sharp enough knife would do the trick, and also a powerful enough bullet. However, a simple arrow will just be deflected upon contact. Also, there are a few places where a regular knife/bullet/whatever you chuck at him could penetrate the skin. Examples include: 1. That place where your two collar bones meet and there is a part of skin with no bone behind it, that leads to your windpipe. 2. Behind your earlobe. 3. The armpits, however, you would require a good angle. 4. The part of your wrist that faces your body when you have your arms down by your side. 5. The eyes, for the more sadistic of the enemies. There are a couple more around the body, similar to the examples given.
Also, whilst he has high intelligence, he is susceptible to mind control. Another weakness is his anger, he can be easily led to get angry, and therefore do things whilst he is angry.
I hope this has helped you.