To get the Katana as an armor permutation in Halo 3, you need to get 1000gp in the game. The new achievements released for Halo don't affect it. The easyst way to get it is to get all the "old" achivements.
you need to achieve all the achievements. you need to achieve all the achievements.
No, they are easier to unlock.
Your work for them.
No
you must have several achievements from HALO 3 and HALO ODST. Along with certain achievements from HALO REACH. Then you will unlock the two helmets
You have to earn achievements or get skulls.
Unlock achievements or play Matchmaking a lot.
Unlock all 49 achievements
you have to unlock achievements
Unlock all achievements in order to get the katana.
Getting the katana sword would mean you would have to unlock every achievement in the game. So you would need both the regular Halo 3 disk for the Campaign achievements and the mythic disk for the online and multiplayer achievements.