The phrase "do down" can have several different meanings, including to criticize or belittle someone, to undermine or sabotage something, or to physically knock someone or something to the ground. Some synonyms for these different meanings of "do down" could include disparage, denigrate, diminish, sabotage, undercut, weaken, debilitate, demolish, overthrow, throw, or knock over. The right synonym will depend on the context and intended meaning of the phrase, but all of these words share the connotation of bringing something or someone down in some way.