fiendishly

adverb2

1. •
   In a fiendish manner; evilly, wickedly.

   • It was hot there too; big flies buzzed fiendishly, and did not sting, but stabbed. — 1899
2. •
   Extremely, very in harsh or negative contexts.

   • Balancing horror and comedy is fiendishly difficult. The two rarely work well together, which is why the successes become so lauded (Evil Dead 2, An American Werewolf In London, Dead Alive). — 2017