So for those that would rather not do the same 20-30 hour grind three times there is another option. You can buy subclass shortcuts in the Xbox live store which will instantly max out your subclass. The only problem with doing this is that the shortcuts are horrendously overpriced at 30$ a piece so I'd only recommend doing it if you either are rich or really hate this game.
Here's a video of what using the shortcut looks like.
Link to store page for Warlock:
https://marketplace.xbox.com/en-US/Product/Destiny-Level-25-...