The phrase "at which" is commonly used in formal writing, particularly when referring to a specific moment or event. However, there are several synonyms that can be used to convey the same meaning in a more concise or varied manner. These include "wherein," "when," "on which," "in which," "during which," and "whilst." Additionally, using prepositions like "at," "in," or "on" with the appropriate noun or pronoun can also be used to replace "at which." For example, "the moment at which" could be replaced with "the point in time when" or "the stage in the process where." By using synonyms and variations, writers can add more nuance and precision to their language while avoiding unnecessary repetition.