Lewis & Short

per-diffĭcĭlis, e, adj., very difficult (class.), Cic. Part. 24, 84: quaestio, id. N. D. 1, 1, 1: navigatio, id. Att. 3, 8, 2.
Sup.: perdifficillimus aditus, exceedingly difficult, Liv. 40, 21.
Adv.: perdiffĭcĭlĭter, with great difficulty, Cic. Ac. 2, 15, 47.