Sentence:  The Prince of Orators directs all Polemics "to exert their Fire and Force on those Points of their Cause which are most solid and plausible, and to glide gently over, or altogether omit those which are weakest and most <WSD>oppugnable</WSD>". 
Meaning of the word marked with <WSD> tags:  (rare) Disputable, contestable, refutable, capable of being oppugned.