To easily slay a dragon, try the following hints/steps:

1. Get good equipament. So far, I'm using the complete Superior Dwarven armor set(also, I use the Dwarven war axe and sword, all Superior).

2. Choose a decent place to kill it. Always look for open and the most plain places, or, somewhere with cover (like a Fort's wall).

3. Attack spot - once you find a dragon, wait for it to land and try to attack its side, avoid the head if it still has plenty of health and try not to attack from behind because the tail deals more damage than the wings. Also, arrows aren't very effective.