use the stinger are javilin
Assault Killstreaks such as the Predator Missile will count towards your Reaper or AC130. Also, the Lethal Support Killstreaks will also work.
you kill it
Kill kill kill !!
Makarav
you kill it with a rs-ass
Yo cant kill him.
When you kill 3 people all at one time.
By Completing challenges
sort of mw3 contains a survial mode where you kill humans rather than zombies
100 bullets kill a juggernaut
you have a good gun and kill everyone you see. :)
The only thing that can kill the grim reaper is his scythe, so that is his only weakness.