Which answer best identifies the word(s) that the underlined phrase modifies?
against an enemy, however, it is used as a weapon of great power.

The underlined phrase ("against an enemy") is modifying the words "is used".

Explanation: "Against an enemy" is a prepositional phrase. A prepositional phrase is a group of words that includes a preposition, an object and, in some cases, a modifier. Moreover, in general, a prepositional phrase modifies a noun or a verb. "Against an enemy" is made up of the preposition "against" and the object "an enemy". Furthermore, it is modifying the verb phrase "is used"; therefore, in this case, the prepositional phrase is behaving adverbially.

the sentence that avoid faulty coordination is:

culture, which is well represented in philadelphia, plays a major role including theatre, music, and the visual arts.  

faulty coordination occurs when the writer gives equal importance two different ideas (clauses) which in fact are not equivalent. in order to correct a misplaced clause or idea, the writer can place less important or emphatic ideas in a relative clause as in the previous sentence.