The best way I can say being able to shorten it effectively. Is what general idea you want to stand out.
If you want the life and death to be the main reason.
(Die fighting, or you will never see the end after.)
Or possibly the soul is the main idea.
(Die fighting, or your soul will never stop suffering)
You mashed two concepts together which makes it complex. Which is not even bad but, in order to hook a reader it has to be simple and easy to understand. i.e When Zack says to Cloud. "For the both of us, you'll live.You'll be my living legacy."
It isn't really neccasary short, but it is more simple. I hope this helps!