The Collateral damage challenge is when you shoot a bullet which kills 2 people. easiest way to get it is with a sniper of some sort as they are usually 1 shot 1 kill.
You do not challenge Infinity Ward
Get into a challenge lobby.
Seatown
challenge lobby
yes
Pick up someones gun and kill them with it
Do the challenge "Ultimate Sacrifice" which you unlock be nuking the enemy while you are losing
complete challenge "silent veteran II"
You have to get invited by a co-host or a host that hosts challenge lobbies
get comando pro 5
You have to get all of the attachments on the ACR to get the mastery.
Yes, but only when you unlock the challenge Flyswatter