What they should have done is,
A: Given Alduin a shout that would banish the spirits aiding you half way through the fight and
B: Given Alduin the stats of the highest level dragon you could encounter at your level with a fortify everything 50% ability. So in essence if you did the quest late enough to encounter ancient dragons he'd be 50% more powerful then an ancient dragon, and with dawnguard the potental for 50% more powerful then a legendary.