User blog comment:Theriotgerald2012/How do i defeat Alduin/@comment-6881629-20130812180705

If you use Dragonrend to early Alduin will glitch and become un-damageable. When you exit the time wound don't do anything (not even equip the shout) until Paarthurnuax says "use dragonrend if you know it!" then you equip and use the shout.

For things like ths your better off asking in the forum.