Board Thread:Wiki Discussions/@comment-3238846-20140422105553/@comment-4912646-20140429210958

The fact is, we don't know if "mastery" of the Fire and Earth natures is required to use Lava Release. Remember that in canon, we've never been told that using all five natures is unheard of, but it has been implied that mastering them all is (like Nagato did). The entire shinobi alliance was able to use a rudimentary Earth Release technique without any prior training, and Kakashi wasn't surprised that Kakuzu could use four different natures, but that he could use high-level (B-rank) techniques of four different natures, which he said requires mastering the natures perfectly.

So maybe, just like the alliance's earth barriers, mixing two natures together only requires the bare minimum level of skill with them. That would explain how Haku and Obito were apparently able to use their "advanced natures" without receiving any sort of training for them. Ever since that earth barrier incident, we really need to reevaluate what it means to use a nature, since that made it clear any average ninja can use low-level elemental techniques.