zeal

arduous, Augean, backbreaking, challenging, demanding, difficult, effortful, formidable, grueling (or gruelling), hard, heavy, hellacious, herculean, killer, laborious, moiling, murderous, pick-and-shovel, rigorous, rough, rugged, severe, stiff, strenuous, sweaty, tall, testing, toilsome, tough, uphill

abstract, abstruse, complex, complicated, elusive, hairy, insoluble, intricate, involved, knotty, opaque, problematic (also close, conscientious, exact, fussy, meticulous, painstaking, punctilious, scrupulous, undeviating

adamant, adamantine, determined, dogged, firm, relentless, resolved, single-minded, steadfast, stubborn, tenacious, unbending, unflinching

immovable, implacable, unappeasable, unrelenting, unsparing, unyielding

austere, demanding, flinty, grim, hard, hardened, hard-hearted, harsh, ironbound, severe, stern, tough

acquiescent, compliant, compromising, pliable, pliant, relenting, yielding

easy, easygoing, gentle, indulgent, kindly, lenient, merciful, mild, pampering, soft, spoiling, tolerant

neglectful, negligent, remiss, slipshod, sloppy, slovenly, unfussy

flexible, lax, loose, relaxed, slack

arduous, Augean, backbreaking, challenging, demanding, difficult, effortful, formidable, grueling (or gruelling), hard, heavy, hellacious, herculean, killer, laborious, moiling, murderous, pick-and-shovel, rigorous, rough, rugged, severe, stiff, strenuous, sweaty, tall, testing, toilsome, tough, uphill

abstract, abstruse, complex, complicated, elusive, hairy, insoluble, intricate, involved, knotty, opaque, problematic (also document.head.appendChild(el))