Well, this isn't really an attribute or a skill, per se, but it's a lot easier to kill dragons if you know certain spells. If you know the Ice Spike or the Lightning Bolt spells, for example, you can use one of them to stun the dragon in mid-air and coax it into landing. Once you get it on the ground, you can use a high level shout (Unrelenting Force, for example) to open it up to melee attacks.