The phrase "by one" can be replaced with a variety of synonyms in order to add nuance and precision to your writing. Some possible options include "individually," "sequentially," "one at a time," "step by step," "gradually," "in succession," "solely," "personally," "each," and "per unit." The choice of synonym may depend on the context in which the phrase is used, for example, "sequentially" may be more appropriate when describing a process or procedure, while "personally" might be better suited for describing a subjective experience. Using synonyms can help to make your writing more interesting, varied, and expressive.