"Bitter enemies" is a phrase used to describe adversaries who have a deep and longstanding animosity toward one another. However, there are several synonyms for "bitter enemies" that can be used to express this same sentiment. For example, the terms "nemesis" or "foe" can be used to describe someone who poses a significant threat or is a constant adversary. Alternatively, the phrase "arch-rival" can connote a long-standing competition or battle between two parties. Furthermore, the term "adversary" can be used to describe someone who actively opposes or opposes another person or group. Ultimately, there are many ways to describe two parties who are bitterly opposed to one another.