Hellishly is the adverb twin of hellish: it tells you how bad or how hard something is. “Hellishly difficult” means painfully, extremely difficult. Same informal feel as hellish — speech, stories, complaints — not a careful report. Though actually people also stretch it into a roundabout “very” (“hellishly expensive”), which is looser than “in a hell-like way.” The idea is still intensity: too much, almost too much to bear.