It wasn't just physical attacks he had to deal with; Han Sen had to battle within the mindscape. There was no skill involved there.
You could lie to others about the power you possessed, but you could never truly lie to yourself. If Han Sen's mind was weak, his nemesis would not have to exert effort in trying to kill him. If Han Sen was weak, Six Paths could just ask Han Sen to kill himself and he'd do what was bidden.
There was a clear difference between the heart and willpower. Willpower improved one's faith, but that was not all.
People with normal willpower and normal body power could still have strong faith.
People with a strong mind might not be the strongest physically, but it was still common for physically strong people to have strong minds.
The fists collided with a sword while faith clanged against faith. There were no sparks for the latter, but the lethality of that dance was just as wicked and dangerous.
