To round up the Halo 3 achievements, we have Annual. This one is a doosy. For Annual, find three friends, fire up the final mission (Halo), turn on the Iron skull, set the difficulty to Legendary, and make your final run on ghosts instead of the usual warthogs. Sounds easy right? It's not. Lucky for us, the guys from the Spartan I Project (SIP) have written an in-depth guide that walks you through how they do annual. I encourage you to read through that guide and watch the videos. Your first challenge is getting to the top of the building. If you're like me, the grenade (or rocket or fuel rod) jump in the guide may be too much to handle. Try it out on Easy a few times to get a feel for it before you attempt it on Legendary with a group. If it proves to be too hard for your group, just work your way up the ramps the usual way. With good teamwork and communication, you should reach the top in 30-45 minutes. If you follow the directions in the guide, you should be up there in less than 10 minutes.
Regardless of how you get to the top, make sure you find the bubble shield and follow the directions in the guide for killing Guilty Spark (basically, deploy the shield by the door when he starts shooting and everyone stay inside it). Once you're back outside, head back down the ramp and find the four ghosts directly under the ramp. If you're not good at maneuvering around things at the edge of the cliff, I'd recommend clearing the pathway to the door on foot. Just don't go through that door until everyone is on ghosts because you'll hit a checkpoint which will teleport everyone, with or without their ghost. Once you've made it to the other side of the mountain, line up in a straight line, and stick to that order. The point man is in charge of navigating while the player at the rear is in charge of telling the point man when to speed up or slow down. If someone tips over, there's usually plenty of time for them to get back in and catch up provided that everyone else slows down. If everyone sticks together, there's a better chance the floor won't fall out from underneath you. That is the most important part to getting to the end. If you can't memorize the path they take in the SIP guide, at least remember to stick together, boost as much as possible, and communicate.
What a strange place to hide a bunch of ghosts.
Of course, you could just ask SIP's Annual Team (private forum, join SIP for access) for help. Their current record time is just 12:26! I tagged along with their team once and we finished in 18:49. Compare that to 1:37:27, which is how long it took Bakken Hood, Husky Trooper, my wife, and I to complete it on our own without any guides. It was brutal. Trust me, you want to enlist the help of SIP, or at least follow their guide. The Annual Team is very friendly and easy-going so don't be shy! They're here to help. We also have a thread in our forums to find help.