When Gwok Jing singlehandedly fought the Golden Wheel Monk, Wan Hak Sai, Siu Seung Tze, Lui Mor Singh, and Ma Gorn Jor in RETURN OF THE CONDOR HEROES, he put up a valiant fight but was eventually overwhelmed by the extremely uneven odds. The Golden Wheel Monk was quite nearly Gwok Jing's equal, and three of the other four martial artists (except for Ma Gorn Jor, who was weaker) were also extremely formidable. One of the reasons that Gwok Jing was overwhelmed was that he fought his five opponents straight up in a direct confrontation. That's very in line with his personality: direct and upfront.
If the same five Mongol mercenaries had fought the other Greats, however, things might have gone differently.
East Heretic Wong Yerk See: Wong Yerk See was a powerful martial artist and also a master of formations and traps. Gwok Jing tried using the Cheun Jen Sect's 7 Stars Northern Dipper Formation to elude his opponents, but was unsuccessful. Wong Yerk See's knowledge of formations, however, was far more profound than Gwok Jing's. It's possible that if Wong Yerk See had been in the same situation that Gwok Jing was in, he would have escaped unscathed by using formations to confuse his enemies.
West Poison Au Yeung Fung: I could see Au Yeung Fung using poison against his enemies. Au Yeung Fung could call out his legions of poisonous creatures to attack the four weaker martial artists, giving him a free hand to deal with the Golden Wheel Monk. One-on-one, Au Yeung Fung has a 50/50 chance against the Golden Wheel Monk.
South Emperor 1 Deng: Possibly, 1 Deng could use 1 Yeung Finger Technique to take out the four comparatively less powerful fighters before engaging the Golden Wheel Monk. I'm not entirely sure that even 1 Deng could pull this one off, however.
North Beggar Hung 7 Gung: Essentially, his approach to the battle would be very similar to Gwok Jing's (possibly with similar results). Maybe the addition of Beat Dog Stick Technique would throw the fight in his favor...at least enough to escape without significant injury.
Central Mischief Chow Bak Tung: Chow Bak Tung's fighting style is unpredictable and whimsical. He could make short work of the four weaker mercenaries, then take on the Golden Wheel Monk in a battle royal.
Ironically, although Gwok Jing might have been the best pure fighter among the Greats, his personality and fighting style might have made him the most vulnerable to the kind of attack the Mongols used against him.