Try using smaller clippers for the legs. Also using a rubber mitt curry to curry the leg firmly for a couple of minutes before clipping it seems to desensitize the leg a little. If you can get someone to stand beside the horse's head and with their fingers lightly "thump" them on the forehead rhythmically, it seems to help for some reason. (That also works for distracting from shots, or with a horse that is antsy with holding a leg up for shoeing. Weird, but I had a jockey teach me that, and it works).
Or have someone hold up the other leg on the opposite side that you are clipping.
This is why I refuse to ever clip my horse's legs (trimming is tolerable, but considering he still sometimes feels the need to kick my teeth in if I use the wrong brush, I don't dare body clip his legs!). Thankfully, he's an event horse, so a classic "hunter" clip is acceptable.
One of the ponies I show clip, though, is just about as bad as my horse is with the clippers. Heavy sedation, an extra pair of hands, lots of time, and small clippers are the only things that make it doable (and even then it can be a rodeo). Choose your sedation wisely...I used rompum (sp?) once on him, and it made his so fast with his feet I could barely get near him. This last time I did him, I used just Ace (though a substantial amount for a small pony), and we did much better, though still not great.
Make sure her legs are VERY clean (use a curry to get deep down to the skin when you wash them), and slick them up really well with ShowSheen or some such product. Also, make sure your blades are sharp and in good condition. All those things will make the job go much faster.
If you are showing in rated competitions, be sure you allow plenty of time prior to a show so that your sedation doesn't test. Depending on what you use and/or the combination you use, you will probably need 7-10 days...but talk to USEF to be sure.
I'm not usually a big fan of sedating for every little thing, but having had some very fast, cranky feet come flying at my head on more than a few occasions, this is one thing I am A-OK with sedating for. It makes it a much more tolerable process for all involved, and since it is one of those things that only needs to happen a couple of times a year (for most horses), it is hard for them to learn that it's not a life threatening situation.