Other points of interest:
- That city has 15 Strength (City States tend to prioritize defensive structures, so it probably has walls). Your Warrior has 6 IIRC and your Spearmen 7. It's like throwing them at Musketmen. Not going to end well.
- Catapults. Catapults. Catapults. There's a reason why everyone is mentioning them. They're siege weapons. Use them.
- Horsemen have an attack penalty against cities (I think it's -50%, although I haven't played unmodded CiV for around 4 months). So your Horsemen would be about as strong as your Warrior against cities. Not good.
- Also, about the turn 127 Warriors thing, I think he's playing on Marathon/Epic. So maybe he hasn't researched Iron Working yet (although you should have Catapults).
Basically, built 1-2 Catapults, whittle down the city's HP with them (you had the right idea with the Chariot/Archer, but they do something like 1-2 damage per turn while Catapults can to 6-7 damage, so it's just faster with Catapults), then march up a Swordsmen (Iron Working)/Pikemen (Civil Service) and take it. It'll be hard and a lot of your dudes might die, but eh. It's not a Maritime CS, so you can kill it. Besides, there's not much Japan can do about it, right?