If you want every achievement then yes.
multiplayer achievements and skulls
Get sculls or get achievements
on an xbox press multiplayer
yes
Halo 2
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.
No
yes everything except multiplayer
There is multiplayer in halo 4
Yes that's the purpose of the multiplayer disk. the odst disk has online multiplayer to.(4 player co-op campaine and fire fight)
Becuase it has amazing graphics and it is multiplayer and of course its online
Yes, Halo Wars is a Xbox Live game, therefore you can play online against other players.