The phrase "hast the upper hand" means having control or dominance over a situation or person. There are numerous synonyms that can be used instead of this phrase, such as "holds the advantage," "is in charge," "commands the situation," "is the leader," "is the boss," "is the winner," "has the upper edge," "is on top," "prevails," and "is dominant." Using these synonyms can add variety and nuance to your language and make your writing or speaking more interesting. It is important to choose the appropriate synonym based on the tone and context of the situation.