Are you trying to clip with scissors, or with clippers? Or have you tried both and she acts the same with either one? You might think that the horse will behave better with scissors than clippers, but that's not always the case so if you haven't tried the clippers, you should try it once. We've got a gelding here that is an absolute pig if you try to trim his bridle path with scissors--but when I use the clippers, he pretty much goes to sleep! (Perhaps he thinks that with scissors I'm going to cut off an ear??)
I agree with Rabbitsfizz--do NOT tie this mare with her head down low. The horse can't rear in this position, but she will still try, and a horse can get very badly hurt by fighting this way. As long as she ties, I wouldn't have a problem snubbing her up--but only with the rope high up. The one gelding (same one that hates scissors) can sometimes put up a tussle over getting a shot, so we have snubbed him up--he'll fuss around & try to squeeze me against the wall, but I get the shot into him anyway. When we snub a horse up, we don't tie him solid--I just take a double wrap around a stout post, then Mom holds the end of it while I give the shot to the horse. If he were to really start fighting (which he never does--he know when we've "got" him) she could give the rope a flip to loosen it on the post & then let go of it, & he'd be free in an instant.