I struggled for quite some time to find a Legendary Dragon myself! When I downloaded the Dawnguard DLC, I think my level was roughly in the high 50's to low 60's, because I made the critical mistake of getting stuck in a certain rut of fighting, using only Heavy Armor with Two Handed weapons for majority of the game, which is why I didn't level up much. When I completed Dawnguard, and unlocked all the related achievements, for both Vampire and Werewolf, I saw the achievement for "Legendary Dragon". I have over 1095 hours logged into the game, I've played the game on Master the whole time, and had over a 100 Dragon Souls, with ever Shout unlocked and completed. I kept encountering Revered Dragons, which was cool, cause it was a new Dragon type to fight, and they're a lot tougher than Ancient Dragons, but I never encountered a damn Legendary Dragon?!?!?! So I started fighting differently, using Light Armor, Magic, Pickpocketing, anything that would level me up.
Level 78 is definitely the magic level to encounter a Legendary Dragon, because with over a 1000 hours of gameplay, I never encountered a Legendary Dragon. After leveling up to 78, I was traveling on Shadowmere, and I suddenly saw a Dragon flying around and automatically assumed it was a Legendary Dragon! So I made a save, rendered the Dragon using the Dragonrend Shout, and it turned out to be a Revered Dragon! I was so pissed off! But one piece of valuable advice that I can give for anyone who's still looking for a Legendary Dragon to unlock the achievement/trophy is that you definitely have to be at least a level 78 to encounter one, and they can only be found in certain areas. They won't appear at any Dragon Altars, nor will you randomly encounter one through your travels. One place that I DEFINITELY know there's a Legendary Dragon, once you've leveled up to a 78 is Arcwind Point. Fast Travel there once you've leveled up to a 78, and I guarantee you, you'll find one, and boy are they tough as hell to beat, especially on Master!
If anyone hasn't discovered Arcwind Point yet, it's located west of the Autumwatch Tower Dragon Altar in the Rift, before Froki's Shack, and South of Haemar's Shame. It's a little hidden and can be a little tricky to find, if you haven't discovered it already, which luckily I had, before I even downloaded Dawnguard.
So I finally encounter my first Legendary Dragon at Arcwind Point, kill it my first try, it was the last achievement I had to unlock in the game, and all of a sudden, I notice that there's 5-6 new achievements, for a new DLC that I didn't know anything about, the Hearthfire DLC!?!?! You can buy plots of land, build and furnish your own house, adopt a child, sounds a little like Fable to me? So I finally thought I could shelf Skyrim, but I'm a completionist, so it looks like I'm downloading Hearthfire, to check it out, and unlock those achievements.
This is my first post, and I registered specifically to lend my advice on this topic. Sorry for the crazy long message, just wanted to be thorough, and hope it turns out useful for the players who haven't unlocked the "Legendary Dragon" achievement. Happy hunting! I'll probably be posting regularly now. Lol