Help needed with the final Japanese scenario

I struggle to find a consistent strategy for the final scenario in the Japanese campaign. It's no problem at all on easy or medium because the enemy sends nothing more than a couple of archers or samurai. However, on hard, they send 2+ archers, 15+ samurai and flaming arrows every couple of minutes.

The main issue with that is that the army can come from two sides, and those two sides are somewhat far apart. When I use only one army stationed in the centre, they generally have time to burn town a castle or other structures, and sometimes their next wave comes from the other side very shortly after the first has been destroyed.

I checked some walkthroughs, but they are all too generic, like this one. None gives concrete advice how to best deal with the large enemy armies. The best strategy I could find is using flaming arrows behind walls, but they are very expensive, too slow to move them between fronts, and easily destroyed should a horseman or samurai get to them. They also cost a lot of pop.

Rushing didn't help either. It's easy to bribe the green faction and then use the ninjas you get to assassinate the leader of the pink faction, but the troops I get aren't sufficient to destroy the red castle, face red's the Hatamoto samuari, and beat their army at the same time.

I also checked Youtube videos, but they are either not on hard mode, or rely on luck (i.e. the enemy sometimes attacks only from the north and not from the east) and seem to struggle too. Any advice on how to beat this scenario consistently?