dd <- data.frame(b = factor(c("Hi","Med","Hi","Low"), levels = c("Low","Med","Hi"), ordered = TRUE), x = c("A","D","A","C"), y = c(8,3,9,9), z = c(1,1,1,2))
sort by columnz(descending) then by columnb(ascending)
1.dd[with(dd, order(-z, b)),]
2.dd[ order(-dd[,4], dd[,1]),]