No Pokemon can solo every battle. If there was, all the top players would use just that Pokemon and nothing else. The Pokemon that gets closest to soloing every battle is probably Rayquaza.
commented 7 hours ago by sumwun
He/She is correct.(I don't know whether she/he is a girl or boy) No single Pokemon is capable of defeating the entire meta. Each and every Pokemon has some counters and checks, so it is impossible to say "Any good Pokémon that can solo battles in National Dex AG". The most viable Pokemon in Nat Dex AG are those who have S Rank i.e. Necrozma-Dusk-Mane, Rayquaza-Mega and Zacian-Crowned. However, each of them also have some counters. Necrozma-Dusk-Mane gets countered by Yveltal and Groudon-Primal, also by V-Create Mega-Rayquaza. Zacian-Crowned can't hit defensive or Weakness Policy Necrozma hard, also Primal-Groudon check it. Mega-Rayquaza gets countered by Zacian-Crowned and Xerneas, etc etc.
So we can finally conclude that there is no single Pokemon capable of doing solo battles in this format. The most capable are Necrozma-Dusk-Mane, Rayquaza-Mega and Zacian-Crowned.