Thats the problem.
Again I say. A battle of attrition against Herc is the worse.
You have to take account how Herc is super nimble despite the size, able to instinctively dodge or block things that are dangerous for him, and has higher physical parameter than most other Servants.
NP aside, tactical retreat aside, even to begin hurting Herc with normal hits require said attack to be above A rank, requires said attack to be good enough to bypass Herc's own AGI and Mind's Eye, requires it to hit vital point and kill him instead of just nicking him in unimportant places, and requires doing it long enough before beaten to a pulp by Herc.