Legendary dragons only start spawning at player level 78 or above, so feel free to use an exploit to level up if you can since this trophy can be on the very tedious side if you play fair just because of the stupid level requirement.
If you don't want to wait until the game randomly puts a legendary dragon in your face, you can go to Arcwind Point, near Helgen. If you're level 78 or above, Arcwind Point will ALWAYS have a legendary dragon waiting for you there. But it'll be alongside many strong draugr, so prepare for a tricky fight.