"Go at someone" is an expression that typically means to aggressively or confrontationally approach or attack someone. There are many synonyms for this phrase, including: confront, challenge, attack, accost, harass, assail, berate, criticize, or rebuke. Depending on the context, some of these synonyms may be more appropriate than others. For example, if someone is approaching someone in order to start a fight or argument, "confront" or "attack" might be more appropriate. If someone is aggressively criticizing or scolding another person, "berate" or "rebuke" might be more appropriate. Regardless of the specific synonym used, all convey the same general sense of a confrontational interaction.