I would suggest getting a set of american (fractional) allen wrenches and using the largest one that fits. The pic isn't very good I assume it's an allen head not a torx. I wouldn't expect it to be tight enough for removal to be an issue.

To Shawn I'll opine that most allen/socket head issues are avoided buy grinding back the wrenchs' tip so it's nice and square edged vs the as delivered rounded sheared end.