Alduin isn't that powerful as most seem to think he is.
Don't forget that he is the first son of Akatosh,the first born,but nothing more.
If you finished the game you should remember that other dragons started doubting his power and feeling like Alduin isn't that powerful as he wanted them to think.
And actually under normal circumstances Alduin isn't that more powerful than the rest dragons at all... It's because he is cheating and travels to Sovngarde and eats souls that he conserves enough power to play it tough.
That's why other dragons call him a lier,as the lyrics of the Main Theme also do.
Alduin wasn't a god neither he was gifted by being the first born as he wanted others to believe.He had the power of a usual dragon and he only became more powerful because he broke rules and ate man's souls.
I'm sure that if in some way the portal to Sovngarde was blocked and Alduin was forced to stop eating new souls,he would be so weak Mehrunes would kill him and then use conjuration spell to bring him back to life and kill him again just for fun..
p.s. I started feeling sorry for Alduin.
Weird eh ?