Remember how I said spam runic attacks? Yeah, don't do that with Spartan Rage. This is because you can't naturally raise the bar unless you're fighting and beating up baddies. It might be most useful whenever fighting a specifically tough group of enemies, and making sure that it's up for whenever that happens is a harsh, but necessary, task. When you do use it, try using it early in combat so that you can go back to building up your bar via beating up the enemies so that for the next fight you might have it ready once more.