User blog comment:Lord Dalek/Alduin...WEAK?!/@comment-70.59.40.205-20150529041702/@comment-6881629-20150529103047

If Alduin isn't taking any damage at all in the Alduins Bane quest then it usually means you used Dragonrend too early, wait until Paarthurnax tells you to use. Its on the quest pages bugs list.