Well, considering bugs, birbs, and fire types are ALL weak to rock, you brought it on yourself, kid.

I really don't understand how that Miltank EVER got its reputation as "That One Boss." The game had practically been THROWING fighting types at you for a little while beforehand. Just grab one of those and get it to a level at least 3/4ths that of the Miltank, and you're golden. Dead simple.