When expressing opposing views, there are numerous ways to express this. The phrase "in flat opposition" can be interchanged with synonyms such as "in direct contradiction," "in stark contrast," or "at odds with." Other options include "antithetical," "contrary," "conflicting," or "opposed to." Depending on the context, words such as "opposing," "different," "discordant," or "contrasting" may also be suitable synonyms. It's vital to select the most appropriate synonym based on the relationship between the opposing views being expressed in the given scenario.