One thing that people don't realize is that Bungie nerfed "normal" and they recommend that anyone with previous Halo experience step up to a Heroic or Legendary, two difficulty levels they bumped up to compensate for normal.
After playing through on coop Legendary and most of single player Heroic, I can definitely say they increased the difficulty.
I also played around on Normal for awhile and it was comical. I beat the first two levels without stopping once to even reload (shit, on the first level, I meleed the entire group of grunts and brutes in that big open area near the cliffs).
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