Archidamus, a Spartan leader, is concluding a speech. After recommending that the Spartans meet the Athenian threat by a combination of parley and preparing for war, then:
You will resolve things most strongly, and for the adversaries most fearfully.
I take the meaning of βουλεύσεσθε to be “you will resolve things”. I’m guessing that there are assumed verb forms for “ταῦτα γὰρ καὶ κράτιστα . . . καὶ τοῖς ἐναντίοις φοβερώτατα.” But I can’t go any further.
Yes, ταῦτα (which you missed in your translation Hugh) is an internal accusative with βουλεύσεσθε, and the adjectives (which I do not take as adverbial but as superlative adjectives) are predicative with ταῦτα.
Steven Lattimore well translates: ”These plans will give you the most strength and make you the most formidable to your enemies.” The language is heightened and dense, making a fitting conclusion to this very Thucydidean speech.