Windshear is a sword in Skyrim that has a chance to knockback enemies that you attack, but due to a bug, the weapon will use the knockback on each attack, so it is regarded as one of the best weapons in the whole game. This is perfect for fighting hard enemies with, especially dragons and boss-like mobs.
In order to be able to get Windshear you need to have completed most of the Dark Brotherhood storyline, as you need to be able to enter the ship (called The Katariah) which is used for the very last mission. If you've already finished the storyline, don't worry, the ship will still be there for you.
So what you need to do is go to the ship and head towards the front of the ship. You might need to clear some enemies on the way there. Walk out on the big wooden pole in front of the ship, but be careful not to fall down. At the end of the pole Windshear will be wedged into the wood, and you can pick it up just like any item.
Now that you have one of the most powerful weapons in all of Skyrim slaying dragon should not be a problem anymore!