Synonyms of arduous in English:

onerous, taxing, difficult, hard, heavy, laborious, burdensome, strenuous, vigorous, back-breaking, stiff, uphill, relentless, Herculean

demanding, trying, tough, challenging, formidable, exacting

exhausting, wearying, fatiguing, tiring, punishing, gruelling, grinding

intolerable, unbearable, murderous, harrowing

informal killing, no picnic

British informal knackering

archaic toilsome

rare exigent