heres what the achivements are like for bungie and 343
Bungies achivments: you can only get them if you download the map packs but you can get them on any map in any halo reach despite what map pack it says at the begining of the achivments discription.
343 achievment: you can only download them with the map pack and you can only get them achivment depending on what pack is on the discription.
obviously bungies was better because you cant get on a zombie game on a defiant map pack and also there is only 2 multiplayer defiant maps so it even harder at atleast bungie let you get the achivment on any map.
soz 343 but bungie are better with multiplayer achivment.