(aka "the Armageddon movie thought too small")
Hmm. If only the example 3554 Amun M-type asteroid was here on Earth. But I don't want to reenact the dinosaur-killing extinction event, and if I'm on Earth at the time then I'm not a spectator but a participant in such an event.
I've got it!
a) mine a convenient C-type (mostly carbon) asteroid to make giant carbon-fiber nano-reinforced glider wings
b) attach the giant glider wings to 3554 Amun
c) FLY IT DOWN TO EARTH
It's going to make a mess of wherever it hits, but pick somewhere big and "empty". 
 Umm, right. "Empty."