library(magrittr)
d <-
  data.frame(
    FW = gl(3, 3, 18),
    Difficulty = gl(3, 1, 18, labels = c("L", "M", "H")),
    E = rnorm(9)
  )
d

# 用 reshape2 package 的 dcast
library(reshape2)
d %>%
  reshape2::dcast(Difficulty ~ FW, fun.aggregate = mean, value.var = "E")

# 用 data.table package 的 dcast
library(data.table)
d %>%
  as.data.table %>%
  data.table::dcast(Difficulty ~ FW, fun.aggregate = mean, value.var = "E")