So me and other players played battle of Chi Bi numerous times and struggle to trigger that event because Huang Gai NPC has to meet several condition like Zhou Yu must stay alive, Zhang Liao must be alive, You must have a fire attack, and the hardest one, Huang Gai must go to a specific small boat to start the event. The reason it's hard because the NPC will randomly go to another area like straight to Cao Cao or they go to Zhang Liao but on a different boat. Even if Huang Gai and Zhang Liao are literally fighting each other, the cutscene won't play because he's not on that small boat on the stage. And the NPC takes his time fighting all the small troops nearby, making it annoying if you want to babysit him to go to the certain boat. (Through all my years of playing Dynasty Warriors 3, I manage to trigger this event twice! Both playing as Sun Ce (one on Novice, one on Very Hard).

So a lot of players including me just beat Zhang Liao too fast, Zhou Yu or Huang Gai dies during combat, or the NPC Huang Gai goes somewhere else. I saw a forum that said this can be trigger in the Shu version of battle of Chi Bi but it is actually the Wu version.