Knives probably aren't long enough to do enough piercing damage to really effect Herc's bulky frame ...... unless he puts them through his skull. Drax loves to dig those knives in....not slash.
Plus as recently posted Supes and Thor are practically immune to the element because their singular biology just rejects it.
Hercs should be the same.....I dunno has Wolvey ever cut Herc?
+ Drax is a destroyer which is an incredible fighting level.
Herc's hardly the most agile fighters, I mean, sure he's not the Hulk in lack of agility, but Drax is very fast and very skilled.
I mean, he handled a brick who was apparently capable of giving Terrax a fight with relative easy, and was able to dance around other Luminals members without being hit once.
Also, I mean, he was able to fight an army of 1000's of Annihilation drones and monsters without being stabbed/sliced, or at least, no particularly visible wounds, and that was just with his two hunting knives from Alaska...