Just the first wave of each chamber is needed to progress to the final chamber. You enter the final chamber by trading the trophies from defeating the first wave to the taru to get key items, and then you can pick the final chamber when you have all 4 key items.
As for it scaling, the dev / community team stated somewhere that the difficulty would vary based on 18/36 man groups. As far as I know, einherjar does not scale.
http://forum.square-enix.com/ffxi/th...ion#post263666
XI Wiki


