The phrase "mighty in battle" refers to a person who is strong and skilled in combat. Synonyms for this term include valiant, heroic, courageous, fearless, brave, gallant, intrepid, and dauntless. These words all convey a sense of strength, bravery, and determination in the face of adversity. Other synonyms might include powerful, formidable, invincible, and unbeatable, all of which suggest a level of physical or mental prowess that is difficult to challenge or overcome. Ultimately, the choice of synonym will depend on the context in which the phrase is being used and the specific qualities that are being emphasized.