You can unlock the event in Story mode, or Free Mode, that's not a problem.The scenes must be unlocked by succeeding or by failing during the objectives of battle. Some battle must be restarted to obtain an alternative scene (stage with star objectives), but not all, some battle need to fail the story objectives.
Only 1 scene belongs to no objective, it is called "narrow ideal
"- force Jin. To obtain it, you must do the stage "East gates battle
", use Zhuge Dan. During the battle, yours officers must die during the ambush. Zhuge dan will say that it's his fault, finish the battle and you will obtain the scene.
Play it in "normal" difficulty minimum, for more security, cause in "easy" yours officers can defeat the enemies more easily, and they don't die, in this scenario, it is not what we want
At the beginning, don't move and let some officers dies, try to keep the morale bar low.