appeal
throw down the gauntlet
defy
dare
glove
quandary
counterchallenge
test
challenge
provocate
run for one's money
challengee
overchallenge
challengeable
superchallenge
daunting
praemunire
mountain
trickiness
challenging
conundrum
fork
mountain to climb
dilemmaticity
battle
hitch
underchallenge
defiance
unmess
doomed if you do, doomed if you don't
tough sledding
rechallenge
challengeful
problem
fix
doo-doo
multichallenge
let
copout
rigour
hot potato
questionable
fork in the road
problematic
hard nut to crack
tough nut to crack
difficult situation
make something of
daring
counterdifficulty