In order to get the metagame achievements with more than one player you have to switch the settings for scoring to free for all and then after you complete a mission it will combined all of the scores and if you reach over 1500 for the missions that require it.
Yes. You can get the game completion achievement through Co-op Mode.
Yes.
yes you can
you can if you play coop campaign mode. The second player will be the arbiter
no you don't you can get the achievement on campaign
Both players will be Master Chief.
Unlock Halo 3:ODST's Campaign Complete Achievement. Once you beat the ODST campaign, visit Halo Waypoint on the dashboard to unlock the item.
you have to do the lendenary 1
The second player will also play as Master Chief in a co-op game.
u dont unlock it it already comes unlocked
You gain achievement points for completing achievements, thus raising your gamer score.
No it does not. You have to beat every level.
score 15,000 or more points on crows nest with campaign scoring on