Is there any way to get a good iv legendary pokemon with the appropriate nature?
You can either RNG abuse in gen 3-5 or Soft Reset in gen 6.
Past generation Legendaries had completely randomized sets of IVs, but the RNG system that determines the details of wild Pokemon is abusable to the point where you can get legitimate flawless shiny Legendaries quite easily if you've gotten the hang of it, and know what you're doing. [
Smogon RNG abuse guide]
Since the RNG mechanics in the generation 6 games of Pokemon aren't as abusable as in the generation 3-5 games, you'd just have to catch/obtain the Legendary normally and hope that the game gives you a good roll. Fortunately in gen 6, nearly all Legendary Pokemon are guaranteed to have 3 Random IVs that are guaranteed to be 31, while the other 3 stats are randomized to be 0-31. For Legendaries you encounter in the wild, you can have a Pokemon with the ability Synchronize in the lead while encountering it, which makes the Legendary Pokemon much more likely (about a 50% chance) of having the same nature as the Pokemon with the Synchronize ability. [
Buckert's Soft Resetting Guide]