δυσκαταγώνιστος
From LSJ
Περὶ τοῦ ἐπέκεινα τοῦ νοῦ κατὰ μὲν νόησιν πολλὰ λέγεται, θεωρεῖται δὲ ἀνοησίᾳ κρείττονι νοήσεως → On the subject of that which is beyond intellect, many statements are made on the basis of intellection, but it may be immediately cognised only by means of a non-intellection superior to intellection
English (LSJ)
ον,
A hard to overcome, Plb.15.15.8, D.H.3.7; hard to refute, Id.Rh.8.3; τὸ δ. impregnability, Corn.ND20.