Ares, the God of War, loses against Perseus, a Mortal Demigod, how? Because he brought Helius to watch the fight, Perseus mentions earlier that what kept him going throughout the movie was coming home to his son, Gods subsist on worship and love from the humans, it's very clear form the beginning that Helius idolises his father Perseus, whereas Ares has few worshipers left due to the twilight of the Gods, so while he is still much more powerful than a mortal, he isn't working at 100% power, whereas he brought Perseus' most devoted worshiper into close proximity, while it wasn't a lot, combined with a childs love for a parent, it was probably giving Perseus a bit of a boost, combined with Pereus and Helius, showing unconditional love towards each other, something Ares, in his mind, has none of, made him sloppy, he clearly has a look of envy in his eyes during the fight.