I'll be studying this text as a part of my work with a VCE-related company. Unfortunately, basically no materials exist at the moment because it's a new item on the text list. When the article goes up and I've finished studying it, I'll be sure to get back to you. Still waiting for the movie to arrive, though.
In terms of preparing for your SAC, I'd try to sort through the main events of the film and try to find a link to what this 'something outside ourselves' is. Once you've established this, you should link between key points of the text that demonstrate this. To improve the quality of your response, explain why this 'something else' is mandatory. I'd also try to challenge the prompt because of how definitive 'cannot' is. Are there examples in the film where someone is considered a 'loner' but is comfortable with who they are?
Also consider the wording of the prompt. I find it noteworthy that it says 'achieving' a sense of identity, as if this is a crucial part of self development. You could either expand on that and say why, or contest it outright.
Apologies that I can't provide anything more specific to Invictus.