"At a time" is a phrase that is typically used to refer to doing something one by one or in a sequential manner. There are several synonyms that can be used in place of this phrase, depending on the context. For instance, the phrase "one by one" can be used instead of "at a time" to denote a consecutive or sequential process. Similarly, "individually" or "singularly" can be substituted to suggest a methodical, individualistic approach. In addition, "gradually," "sequentially," and "progressively" can be used to indicate a process that occurs gradually over time, rather than in a single instance.