You play a lot of matchmaking games and over time you gain XP and rank up. This requires Xbox Live, if you didn't know. (If you didn't know that, that is very sad)
You get exp by winning matchmaking games.
you need to have 100 exp and 10 skill level to become a lieutennant (officer) on halo 3
You win games to earn EXP.
keep doin matchmaking.
Go to Halocharts.com to see
if you lose too much on a ranked playlist in matchmaking you can lose exp
100 exp. and a skill of at least 10
Grifball is a Double EXP Weekend playlist on Halo 3 matchmaking. It resembles football, but with bombs, swords, and Gravity Hammers! Grifball can be played in Halo 3's online multiplayer whenever it is featured as a Double EXP Weekend playlist.
yes if the exp goes under the exp that you needed to go up a grade u will be degraded
yes you can
you go up exp by winning and lose exp by quitting
I expect Bungie to keep the extras from halo 3 and move them to reach. So, to answer your question yes I think they will have double exp. weekend in reach.
You have to get achievements when you playing with a Live account.