Legally, by completing the game's challenges and unlocking all weaponry, titles and emblems. There are illegal ways of doing this to, such as hacking, or modding to make it easier to play. If you want to feel like you have achieved something, do it the legal way, like me.
if its the map packs you need microsoft points if your talking about career you cant get those maps
The doors at target open automatically...
You unlock it at level 36.
By either completing all the challenges available in the Barracks section, or by hacking.
You cant.
No.
you have to reach level 50
Get to Prestige 9.
you can unlock it with you're first killstreak award unlock
You unlock it by getting to 10th Prestige.
you cannot unlock them you have to buy them on PSN or LIVE Marketplace
Sorry there is no alien invasion
Level 38
Level 48