challengingly
multichallenge
pain point
rigour
importantly
pull off
rub along
rough sledding
hard time
Matterhorn
possibly
boil the ocean
make the best of a bad bargain
pull teeth
have a mountain to climb
can
hit
crucially
mountain
challenge
eke out
daunting
otherwise
with difficulty
get out
arduously
resource
vitally
tough nut to crack
fragilely
hard nut to crack
hard-set
problem
mission impossible
lightning in a bottle
enmesh
trouble
hard yards
power
underchallenge
much
stumblingblock
hard
superchallenge
considerably
hardpressed
inaccessible
trickily
an old dog for a hard road
substantially