Just progress through the main story and travel to Sovengarde which where you will find it all foggy.Find your way to the Hall of Valor speak with the three hero’s who shouted Alduin to the past with Dragonrend.Exit the Hall of Valor then use the Clear Skies shout to get rid of the fog.It will dissipate then kill Alduin.
Strategy:Use Dragonrend on him then just go ham on him.