You have to complete the Pavelow III challenge, which I believe is call in 25 Pavelows.
Complete the Flawless challenge Survive the entire game see related link
You have to bomb the entire enemy team with a single airstrike. It could be a stealth bomber, which is easier.
There are 3 'weed title's' available in Call of Duty: Modern Warfare 2: Joint Ops: Get 15 assist kills (hurt an enemy player and have a teammate kill him) High Command: Become first prestige, level 45. Blunt Trauma: Get 2 melee kills while you have the Riot Shield equipped. Next to that, you also have a weed emblem: http://images1.wikia.nocookie.net/__cb20091126044537/callofduty/images/4/43/Emblem-weed.jpg To unlock this, you need to complete The Denier challenge.
In order to complete the cluster bomb challenge, you have to kill 5 enemies with a single air strike. I suggest using the stealth bomber with UAV for this. A recommended game type would be ground war.
i think this is when you destroy certain amounts of the enemies tactical insertions.
complete challenge "silent veteran II"
you dont unlock it at a level you unlock it by killing somone right before they get a 10 kill strick and you will earn the pot leaf emblem
complete challenges from the barracks , gennerally the harder the challenge the cooler the title
Complete the "Omnicide!" challenge which is killing the entire enemy team in 10 seconds.
Get 50 kills with an AC130. You must be prestiged in order to complete this challenge.
You do not challenge Infinity Ward
You have to complete the 'Rival' Challenge where you kill the same player 5 times in a single match.
You have to complete the Pavelow III challenge, which I believe is call in 25 Pavelows.
Complete the Flawless challenge Survive the entire game see related link
Complete the Challenge "Ultimate Sacrifice". Call in a nuke while your team is losing.
Get into a challenge lobby.