Find and claim this skull when playing on Normal, Heroic or Legendary difficulty.
Effect: "You’ll miss those eyes in the back of your head." -- Your motion sensor disappears.
As you are walking down from the anti-air gun you destroyed in the previous mission, you encounter a ramp (next to a missile launcher). Around this ramp, you hit a checkpoint. At this point, you should also hear a marine yelling, “There! Over There!”. Look up and to the right, directly at the roof of the building next to the missle launcher. There is a single flood form (not to be mistaken with the two other flood forms jumping in front of you) which is holding the skull. Kill him before he jumps, and he will drop the skull down to the ground where you can retrieve it. Too early, and the skull will get stuck on the roof though, so just time it carefully.
Argh, I'm trying to get this achievement, but it's becoming so difficult. I'm following the Halo Wikipedia's advice here... http://halo.wikia.com/wiki/Halo_3_skulls ...but it's a little confusing, because there are TWO (2) Flood forms, and I don't know which one to shoot the arm off of (and I can't even see the skull being carried by either one)! Does anyone have any tips or advice? (I'm this close to asking if anyone would like to help me. Yes, I'm asking for someone to take a few moments just to shoot the darn thing to get the skull.)
Use this guide http://uk.faqs.ign.com/articles/823/823401p1.html You don't need to hose the guy though. 1 Player, with well timed shot (as he pauses before he makes his jump) and he drops the skull in the street. (can also drop it on the roof and you have to try again... think this happened when i shot him too soon, ie as he was running up to his pause position).
I literally replayed this same section for 5 hours trying to "time" my shot correctly. from the first checkpoint it only takes 10 seconds for the flood form to show up so, yeah I replayed this section over and over nearly 1800 times. I was so completely frustrated that I was searching far and wide on the internet looking for any additional tips or insights. I figured I had about every single millisecond of timing tested out. Finally I came across someone who had the misfortune of not realizing they were on Easy difficulty after their 200th try...congrats my friend, you are approximately 9x smarter than me. If you are struggling with this achievement do yourself a favor and double check your difficulty.