Synonyms for Gruelingly:

adv

•hard (adverb)
distressingly, agonizingly.
•painfully (adverb)
offensively, aggravatingly, arduously, smartingly, miserably, Insufferably, piercingly, distressfully, sorely, odiously, irksomely, witheringly, bitingly, oppressively, unpleasantly, bitterly, gnawingly, distressingly, cruelly, uncomfortably, hurtfully, acutely, disagreeably, painfully, annoyingly, achingly, horrendously, tryingly, agonizingly, unbearably, severely, tormentingly, grievously, torturously, stingingly, woefully, troublesomely, excruciatingly, irritatingly.

adv.

•hard (adverb)
distressingly, arduously, severely, agonizingly, painfully.
•painfully (adverb)
offensively, aggravatingly, arduously, smartingly, miserably, Insufferably, piercingly, distressfully, sorely, odiously, irksomely, witheringly, bitingly, oppressively, unpleasantly, bitterly, gnawingly, distressingly, cruelly, uncomfortably, hurtfully, acutely, disagreeably, painfully, annoyingly, achingly, horrendously, tryingly, agonizingly, unbearably, severely, tormentingly, grievously, torturously, stingingly, woefully, troublesomely, excruciatingly, irritatingly.