Yeah, hit him while he's airborn, idealy with a lightning spell like Chain Lightning, and it will crash to the ground, I didn't know that could happen the first time I shot one down with my bow and actualy yelled a rather naughty word when he flew right at me then smashed into the ground like the war-rhino The 300. Gave me a helluva fright, and actualy caused terrain deformation-the gouge in the ground wasn't just texture, there was actual terrain def, I was quiety impressed. its not QUITE terrian deformation as you'd think, its actually an additional bit of land that get added under the dragon as it slides along the normal ground, so it appears to be ripping up the erath with its body, only reason i know this is because a dragon crashed when i was fighting it on a flat stone walkway of a barrow and two things gave it away, a rock clipped through one of the edges, spoiling the illusion and also the ground was grey stone and the 'carved' up bit was earthy brown...having said that, when it happens somewhere like the plains outside whiterun it does look truely amazing to see such a powerful beast crash and leave a huge gash in the ground, especially when it's killed shortly after and it skeleton lies at the end of the gash
