I have done well by doing two things. To get rid of some of the 'sheer weight' of a long, thick mane, I use the clippers on the UNDERSIDE of the mane, 'roaching' off as much as half the overall width of the mane. This is simple and easy to maintain with good electric clippers. To simply thin and shorten, I do use thinning scissors....a basic 'human' kind. Yes, it is slow and tedious, but if you are careful and don't try to hurry the process, it can work very well. Have mane clean and well-brushed out first, then part off a SMALL strip of hair, hold it straight out, 'cut' about three times at different locations along the length of the hank of hair you are holding, at different locations. Repeat along the entire mane. You may need to go through and do the entire mane again in the same manner, esp. if you want to shorten it significantly. Yes, it will take some practice to do it well...but remember, hair grows back!
I will NOT 'pull' a mane....the horses HATE it(how would YOU like to have your hair thinned/shortened by yanking out a few hairs at a time??)...and it makes your fingers VERY sore. I've not tried using the 'clipper blade' thing, but seems to me it would be more damaging to the hair than the straightforward
cut of scissors. JMHO. On reflection,I recall that if I wanted to both loose some of the sheer weight of a mane, AND shorten it, I did the 'underclipping' first, then used the thinning shears...just need to remember that it might take less 'cuts' w/ the thinning shears after an undercutting, as you working w/ much less volume of hair! Good luck!