The Legend Achievement - The Elder Scrolls V: Skyrim - Dawnguard
This Achievement is awarded for defeating a Legendary Dragon -- but those do not begin to appear with any reliability until you have leveled your character to a minimum of Level 78 -- so if you are not at or near that level yet, you will want to back up and read the Ability/Level Grinding Section of this guide and partake of its time-saving goodness.
Once you have reached Level 78 finding the Legendary Dragon is not as hard as you might think... One way to do it is to head to the Dragon Markers on the map where you previously fought dragons to clear them, as they have a higher chance for a random encounter. Another is to simply let it happen when it happens...
Either way when it does happen you will have something of a battle on your hands, because these Legendary Dragon are not push-overs! Be prepared to heal, be prepared to dish out as much damage as you can, and it may be an idea to have more than a few potions on hand just in case!
Battling with a Legendary Dragon
As far as tactics go battling these guys is really pretty much the same as any of the other higher-level dragons -- the trick is to get them on the ground where you can get at them with melee weapons... As you can see from the video here, while it was attacking from the air I was at a significant disadvantage and I needed to find a place wide enough for it to land. Once I did that, heading in to hack-n-slash was the way to go, and I quickly had it dead and the Achievement unlocked.
My advice about bringing potions with you was not casual (I was serious) and as you can see from the video I ended up needing a few myself.
Rumor has it that these guys can drop some fairly desirable loot -- that was not the case with the one I killed but I suppose anything is possible, right? Bearing in mind that it is not like you will only ever battle one of them -- they become the top tier of dragon that you will encounter in your random encounters with dragons, so I would imagine that you will see more than a few as you play through the expansion.