There's a seventh boss. Dodge his fireballs and reflective eye beams, shoot the tiki dancers as necessary, and pound away until Zhaka Mu reveals his second form. This form works much the same way as the first.
Once you defeat version two, get ready for the third and final form! This part's quite easy because you can frequently hit your foe with two shots in a row, unlike most boss battles. Shoot quickly and save the day! You'll unlock the final avatar award, Boss Rush mode, and of course, a tasty Achievement.