This question already has an answer here:
What degree and types of force is person A allowed to exert against person B to avoid battery by B? Does it matter whether the battery is threatened or occurring? What would be some guidelines for protecting ones self yet staying within the law?
The state legals codes I've read say, more or less, 'what is necessary', but surely, even if the only weapon a weak 5' 100lb person has is a gun they are still not allowed to shoot, possibly kill, an athletic 6' 200lb person in self-defense against battery such as in the link below.
If this general question is too general to answer, my apologies, then please substitute the following real-life situation as the scope:
Note that this question is also related to What is the basis for how aggressive a person can be in self-defense?, which is a question about theory, 'What is the basis', and regards self defense against any level of threat (not limited to battery). Moreover, that question elicited an answer arguing that person A is not allowed to use any force because a reasonable person would not be uniformly adverse to hugs to begin with. However, according to this question What if any law is broken if Person B hugs Person A against A's expressed wishes?, a bodily crime is being committed, and common sense would be that one has the right to physically defend one's self against a bodily crime. So either the answer is incomplete/wrong, or that question is so malformed as lead to not being interpreted as intended.
However, I find my questions answered https://legal-dictionary.thefreedictionary.com/battery, a resource I was not aware of prior to following @DaleM's link to https://legal-dictionary.thefreedictionary.com/Reasonable+Force