Herculean
abstruse
arduous
brutal
complex
critical
delicate
demanding
difficile
difficult
exacting
formidable
hairy
hard
hard-earned
hard-fought
intricate
knotted
knotty
laborious
mean
no picnic
not easy
operose
polysyllabic
rigorous
rough
rugged
sesquipedal
sesquipedalian
set with thorns
severe
spiny
steep
strenuous
thorny
ticklish
toilsome
tough
tricky
uphill
wicked