Straight up, Lee Mok Sau has no chance whatsoever in a fair fight against any of the Greats of L/ROCH. The martial arts skills and inner power of each of the L/ROCH Greats were far superior to those of Lee Mok Sau. Lee Mok Sau's threat, however, was not so much her martial arts skills (as formidable as those were) as it was her arsenal of poisons. Her Ice Soul Silver Needles could do catastrophic harm (and often did).
The Greats' fighting instincts and reflexes had to be among the very best of their era. They *had* to be, or how else would these individuals have qualified to have been the Greats? That being said, do the Greats really have any defense against Lee Mok Sau's poisonous weaponry? I can see West Poison Au Yeung Fung being immune to the poison, of course, and Gwok Jing had limited poison resistance due to his consumption of Leung Tze Yung's snake's blood. But East Heretic Wong Yerk See, South Emperor 1 Deng, North Beggar Hung 7 Gung, Central Divinity Wong Chung Yeung, Lam Chiu Ying, Kau Cheen Yan, Chow Bak Tung, Yeung Gor, and the Golden Wheel Monk had no such immunity or resistance to poisons. If they get stricken by the Ice Soul Silver Needles, they will be affected. As the Golden Wheel Monk proved, Greats-level martial artists had the inner power to purge the Ice Soul Silver Needles' poison if necessary, but in an actual fight, Lee Mok Sau would now allow a Great to sit down and circulate his/her inner power to expel the poison (she would follow up with another attack). Given this, is a fight between Lee Mok Sau and any one of the Greats really as one-sided as it seems? Although Lee Mok Sau would die in an all-out fight against a Great, isn't it conceivable that she could take the Great with her?