The flood have an answer in Orbital Bombardment. The rings need humans to activate them (or forerunners) but are a last resort in case everything needs to die. Force Ghosts also can manipulate things in the physical world on a very massive scale. What part of "more powerful than you can possibly imagine" does nobody get? In universe instances of Luke Skywalker's force ghost collapsing stars and remaking solar systems. Halo rings wouldn't exist for very long, especially considering the forerunners would not wish to use them giving any cohesive force a chance to neutralize them. The death stars (all 3) would be immaterial to the battle, as would the halo rings. Both sides have the ability to effectively negate any super weapon pretty much at will. I'd say the only real "super weapon" that would have any impact would be the Eclipse class super star destroyer. But that would likely get destroyed as well.