I’m a halo player with 920 GS on Halo 3, can anyone walk me through the achievements so I can get the katana and security? (No, not boosting)
Try TrueAchievements. Plenty of people looking to have sessions for unlocking achievements.
"The Katana can no longer be unlocked by just earning 1000G in Halo 3. One of the only ways to unlock the Katana after the 1.2 update and 1.3 is to unlock 49 out of the 79 achievements. Because unlocking only 40 of the 79 achievements is worth 1000G, the conclusion is that players must have 49 achievements to unlock the Katana. The Katana, as well as the Security permutations, are fabled “armor” glitches, in which they appear and disappear randomly. With the Security, a 1000 gamerscore locks it in, and for Katana, 1750 guarantees it. Since the release of the Mythic Map Pack, the Katana seems to unlock for those who download early (without the need for 1000 gamerscore OR 49 achievements)"
According to a quick Google Search. TrueAchievements and any other Achievement Hunting type group are your best bets