Find the Synonym of Demanding and Challenging Words

algorithm, arduous, ascetic, austerity, benchmark, challenging, climb, complex, complicated, compulsory, crucible, degrade, demanding, dissertation, drudgery, evade, evasive, excruciating, exigent, finicky, formidable, grueling, implacable, insist, insistence, insistent, insolvent, insuperable, insurmountable, intransigent, laborious, mandatory, meteorology, obdurate, obligatory, onerous, premature, protester, Pyrrhic, rapacity, rapid, rebel, rebellious, recalcitrant, requisition, stringent, stubborn, summit, surpass, taxing, toil, travail, unapproachable, undeterred, unfathomable, unrelenting