The phrase "in/at one fell swoop" is commonly used to refer to an action that is done all at once or in a single, sweeping motion. Synonyms for this phrase include "in a blink of an eye," "in a flash," "in an instant," "at once," "all at once," and "with one stroke." Other similar phrases that convey the same idea include "in one go," "in one fell sweep," and "in one shot." These synonyms are particularly useful when trying to convey a sense of speed or suddenness in writing or speech, and can help make language more dynamic and engaging.