Counter for Korean War Wagon

I think the simple answer is

Get in close to nullify the war wagon's range attack advantage. Like all cavalry units, it is susceptible to pikemen, skirmishers and camels.

Here are some more tips, but it basically comes down to the same point

Pikemen are especially effective if they manage to get close. A practical approach though, is to use cavalry - especially light cavalry which are somewhat less susceptible to missile damage. Cavalry can close in fast and quite effective in groups.

Massed war wagon are pretty hard to beat and will take down much of your pikemen or cavalry before they get close. One alternative approach that I've seen used (but never used myself) is to use a small group of onagers. The onagers are kept behind other units for protection so they don't get attacked right away, and they all attack at once once they get in range. This only works if they are fully upgraded, and even then this is basically a sacrificial strategy, you you will end up losing your onagers to the war wagons that survive the initial attack. But it does destroy a number of war wagons and weaken the others, and give a better chance for other units to close in.

if possible use cavalry men, that is the best way to finish off a war wagon..

Use mass Skirmishers, preferably with Onagers behind them. If required, some Light Cavalry can also be expended to soften them up. I feel this is the best approach keeping in mind the costs involved and the fact that any civilization can implement this counter..

Keep in mind that you will lose quite a few units, but I have used this several times, and think it works best. Skirmishers are easily replaceable. And since War Wagons are basically (very) beefed up Cavalry Archers, you get a neat bonus while attacking them