adjective

needing much effort or skill to accomplish, deal with, or understand.

she had a difficult decision to make

hard,strenuous,arduous,laborious,heavy,tough,onerous,burdensome,demanding,punishing,gruelling,grinding,back-breaking,painful,exhausting,tiring,fatiguing,wearing,wearying,wearisome,hellish,killing,knackering,toilsome,exigent,problematic,puzzling,baffling,perplexing,confusing,mystifying,mysterious,complicated,complex,involved,intricate,knotty,thorny,ticklish,obscure,abstract,abstruse,recondite,enigmatic,impenetrable,unfathomable,over one’s head,above one’s head,beyond one,fiddly,sticky,no picnic,gnarly,wildering,involute,involuted