Well, for starters, to divine as a verb means to find water with a rod or stick. It is an old, mysterious and unscientific method that has been used to locate underground water sources, when people need to dig a well somewhere. It doesn't work very well,or really at all. And it doesn't work very well in this usage where I think it is intended to suggest a psychic ability to know what another person knows, or guesses or imagines... Anyway, to answer you question, the first phrase means 'how strongly she would resist him' or 'how strong her resistance to him would be'. The second phrase refers to the energy he had in 'reserve' to do any kind of battle with her resistance to him. Can I ask where you took this from?

In this case, he knew that her resistance was weak. You could think of "strength" in this case as the "level" of her resistance or ability to resist (it's neither strong nor weak by itself, it's the context that will tell you). "Reserves" would be the "amount" or degree that she can withstand or resist before giving in to the thing that she's resisting.