1. 
[syn: arduous, backbreaking, grueling, gruelling, hard, heavy, laborious, operose, punishing, toilsome]
The Collaborative International Dictionary of English v.0.48:
grueling \grueling\, gruelling \gruelling\adj.
   1. characterized by toilsome effort; requiring extreme
      physical or mental effort; as, a grueling campaign.
   Syn: arduous, backbreaking, back-breaking, exhausting,
        gruelling, hard, heavy, laborious, labourious,
        punishing, slavish, strenuous, toilsome.
        [WordNet 1.5]
WordNet (r) 3.0 (2006):
grueling
    adj 1: characterized by effort to the point of exhaustion;
           especially physical effort; "worked their arduous way up
           the mining valley"; "a grueling campaign"; "hard labor";
           "heavy work"; "heavy going"; "spent many laborious hours
           on the project"; "set a punishing pace" [syn: arduous,
           backbreaking, grueling, gruelling, hard, heavy,
           laborious, operose, punishing, toilsome]
Moby Thesaurus II by Grady Ward, 1.0:
60 Moby Thesaurus words for "grueling":
   Herculean, annoying, arduous, backbreaking, besetting, bothersome,
   burdensome, castigatory, chastening, chastising, corrective,
   crushing, devitalizing, disciplinary, draining, effortful,
   enervating, enfeebling, exhausting, fatiguesome, fatiguing, forced,
   hard-earned, hard-fought, heavy, hefty, inflictive, irksome,
   killing, labored, laborious, onerous, operose, oppressive, painful,
   penal, penological, plaguey, punishing, punitive, punitory,
   retributive, sapping, strained, straining, strenuous, stressful,
   tiresome, tiring, toilsome, tough, troublesome, trying, uphill,
   vexatious, weakening, weariful, wearing, wearisome, wearying