arduous, Augean, backbreaking, challenging, demanding, difficult, effortful, exacting, formidable, grueling(or gruelling), hard, heavy, hellacious, herculean, killer, laborious, moiling, murderous, 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 problematical), recondite, serious, spiny, stubborn, thorny, ticklish, trickybruising, burdensome, exhausting, labored, onerous, oppressive, stressful, taxing, tight, tryingannoying, bothersome, distressing, irksome, troublesome, vexatiousgrievous, grim, strict, stringentbrutal, cruel, inhuman, painful