Sorry, but there is no defending this. Alduin, is the very definition of an "anticlimix boss".
The issue, is one of inconsistency. What the game tells us about Alduin, contradicts what is given to us in battle. When he fought the three heroes of Sovngarde, they were brutally destroyed by him when they were still alive, and at the prime of their life...even with all three of them shouting him down with dragonrend. When they aid us to fight him, they damage him, and tank him better than they did when they were even alive.
He was meant to be the opposite end of the prophecy. He should have been, in every way, our character's antithesis, he being our bane, just as much as us being his. A duality trying to cancel itself out. Instead, the entire fight is stacked in our favor, in a way that diminishes Alduin as a character and threat, and that diminishes the meaning and significance behind our character's own supposed legend in the making.
What was supposed to be our character's defining moment, felt weak and lackluster. We did not fight a titanic struggle against a being every bit our equal or greater, with the entire fate of existence hinging on our desperate struggle. We stamped out a speed bump in the path of our almighty character, because there is nothing that can challenge us.