Adverb
as a devil; in an evil manner
Source: WordNetCancer is such a ruthless adversary because it behaves as if it has its own fiendishly cunning agenda. Paul Davies
Men have fiendishly conceived a heaven only to find it insipid, and a hell only to find it ridiculous. George Santayana
his writing could be diabolically satiric Source: Internet
According to WhatsApp, it’s because of a technical knot that’s fiendishly difficult to unpick. Source: Internet
As far as I am concerned with pizza simplicity is best and freshness paramount and you'd better have a fiendishly hot oven too. Source: Internet
Because of varying domestic rules on tariff retaliation, coordinating any action between several countries "quickly becomes fiendishly difficult," the diplomat said. Source: Internet