“Grip the long sword with a rather floating feeling in your thumb and forefinger, with the middle finger neither tight nor slack, and with the last two fingers tight. It is bad to have play in your hands.
When you take up a sword, you must feel intent on cutting the enemy. As you cut an enemy you must not change your grip, and your hands must not "cower". When you dash the enemy's sword aside, or ward it off, or force it down, you must slightly change the feeling in your thumb and forefinger. Above all, you must be intent on cutting the enemy in the way you grip the sword.”

Go Rin No Sho (1645), The Water Book

Go Rin No Sho (1645), The Water Book

Letter to William Windham (30 December 1794), quoted in R. B. McDowell (ed.), The Correspondence of Edmund Burke, Volume VIII: September 1794–April 1796 (Cambridge University Press, 1969), p. 104
1790s

Go Rin No Sho (1645), The Wind Book
Context: Whenever you cross swords with an enemy you must not think of cutting him either strongly or weakly; just think of cutting and killing him. Be intent solely on killing the enemy. Do not try to cut strongly and, of course, do not think of cutting weakly. You should only be concerned with killing the enemy.

This is sometimes attributed to Sun Tzu in combination with the above quote, as well as alone, but it too has not been sourced to any published translation of The Art of War, though it is similar in concept to his famous statement in Ch. 3 : "It is said that if you know your enemies and know yourself, you will not be imperiled in a hundred battles..."
Misattributed

Go Rin No Sho (1645), The Ground Book

1950s, Loving Your Enemies (November 1957)
Context: Another way that you love your enemy is this: When the opportunity presents itself for you to defeat your enemy, that is the time which you must not do it. There will come a time, in many instances, when the person who hates you most, the person who has misused you most, the person who has gossiped about you most, the person who has spread false rumors about you most, there will come a time when you will have an opportunity to defeat that person. It might be in terms of a recommendation for a job; it might be in terms of helping that person to make some move in life. That’s the time you must not do it.

As quoted in A Galaxy Not So Far Away : Writers and Artists on Twenty-five Years of Star Wars (2002) by Glenn Kenny, p. 99

Source: The Black Company (1984), Chapter 6, “Lady” (pp. 192-193)