Hey TTG upon finishing the campaign level "Assault on the Control Room" on co-op Legendary with a friend of mine, we somehow unlocked the achievement "He's Unstoppable". In order to obtain this achievement you have to complete any level of Halo CE on Heroic difficulty or higher without taking any health damage, and just so we are clear my friend and I died at least 15 times each but we both got the achievement after finishing the mission. Obviously this achievement is difficult to get considering that you can't take any damage whatsoever. So I'm curious, did we do a glitch that we are unaware of, or is the game glitched? Either way we now have the achievement. Try this out yourself, all we did was start up the mission on legendary and of course finish the mission, just the two of us. Maybe you and a friend can unlock the achievement exactly how we did, without all the frustration of resetting the level because you got shot. Hopefully this helps, I'll try to answer any questions you have in case I didn't explain this clear enough.