Doggie downers for clipping my dogs nails?

I have a big 100lb dog and he absolutely hates getting his nails clipped. He refuses to let anyone do it. I've tried to slowly get him used to the clippers and I've tried to give him natural doggie downers and even benedryl but nothing seems to have any affect what-so-ever. I've also taken him to a groomer and it takes forever to get his nails clipped because hes so strong so he thrashes about and jumps and sometimes scratches and tries to bite the groomer. Does anyone have any suggestions on what would be a good doggie downer just so I can get his nails clipped?