If Thanos has prep, sure he can win.
But without? He's screwed.
Ares doesn't even have a physical body. The guy with scary blue armour and huge battle axe? That's just a projection of himself. So is the blond guy with a scars on his chest.
He's quite literally a god. He can create and unmake reality. He can create universes and destroy them in a second thought.
He's also absorbed Hades' powers making him doubly powerful.
Thanos can't really attack him. One tactic Ares likes to use is to become 'one' with the universe and just blitz his opponent from a higher plane.
And any act of violence against Ares just makes that much stronger. He feeds it on it. That's what gives him life.
So, Thanos can blast away all he wants. He wouldn't be harmed in anyway. Ares is the personification of war and conflict. Unless war and conflict across the multiverse dies out, Ares is immortal, infinite and unstoppable.