I highly recommend Dona Negra at Parador #5. She's one of the original guide/horse services.
Just a word of warning: the hike there can be very, very muddy except in the height of a drought. The first time we went we were told we should go by horseback. I thought we were being scammed but Alida said we should, so we did.
Half way up we saw groups of Euros with somber "This Is Not Fun" faces hiking down, completely caked with mud below their belts.
I was glad to be on horseback.
December is a wet month. Consider yourself warned...