First off, I agree that Linghu Chong should not have been able to even stand given his condition. His accuracy and speed also seem too ridiculous to be true.
However, it's not entirely impossible if you give Jin Yong some benefit of the doubt.
Here's why (translations thanks to Lanny):
Before LHC fought the 15 masked men, 5 had already been wounded by the Yue couple, so LHC wasn't up against 15 completely fresh fighters.
Madam Yue had given one guy a heavy chop in the leg; Yue Buqun had cut off one guy's arm and thrust his sword into another's leg.
Next, Yue Buqun alone held against 8 men, while 7 pointed lanterns at his eyes to distract him.
"fifteen masked men surrounded Yue Buqun in a circle. Eight of them took on eight different directions to attack Yue Buqun simultaneously..."
Now, were you able to suspend your disbelief of YBQ fighting 8 people simultaneously? Who is he, Nezha?
"...another enemy got stabbed in the shoulder and had to drop his iron staff....
Yue Buqun had plenty of inner strength...the longer the fight lasted, the more energetic he appeared to be." YBQ also uses a palm to break another's ribs and knock his cane out of his hands.
When fighting LHC, the 15 masked men "threw their torches on the ground...their body portion above their waists remained vague in the dark...
the fifteen masked men approached forward half a step at a time, no one dared to launch the first attack..."
So now, 15 men, with 5 wounded, advanced half a step at a time. Not only that, but they probably couldn't see LHC very clearly and each was apprehensive of him.
LHC has the initiative, and the 15 of them are practically standing still for him. Now, Lanny translated "一瞬之间" to "within a split of a second," but it could just mean an indefinitely short time, not necessarily a fraction of a second.
Let's say he targets the 10 unscathed ones first:
Assuming LHC has astounding speed and accuracy (this is Wuxia after all)
You could imagine him blinding 2-3 before the rest even register what's happening. 5-6 would be blinded before they had proper time to defend or jump back, and the rest would be too shocked to put up much of a resistance. It's dark, they're frozen in shock from the unexpected attack, and 1/3 of them are wounded.
Even with this defense, I admit that I thought it was unbelievable when I read it. However, think about Duan Yu and his lasers, people scaling vertical cliffs or fighting armies and escaping from hundreds of archers unscathed, shooting blasts out of their palms...