Much like they did with Arkham Knight, Warner Bros. have made the decision to lock the true ending of Shadow of War behind a lot of end game activities. Unlike with Arkham Knight, the end game activities here revolve all around the same mode, and depending on how you feel about metagames, are actually fun.
That said, getting the True Ending (which is basically completing Act 4) unlocked will require a significant amount of time- we are talking of at least a 5-6 hour investment. You can try and expedite the process by using the in game microtransactions- but it is advisable that you just spend the couple of extra hours and try to get this done for free.
In short, here are the steps:
- You need to complete Siege Quest. This entails defending all the Fortresses, and reaching Stage 10 (yes, there are 10 stages of defense). Doing so unlocks the True Ending.
- It is recommended you invest in defensive upgrades, because later stages see the challenge upped, and the enemies become more aggressive. You will automatically reach level 60 during Act 4.
- Target the Assault Leader- get him out and you defend against the Siege.