How much health do you have?
I found Alduin pretty easy, personally. He didn't get his fire onto me that much, though. Once I could get him on the ground, I just did what I do with most dragons - hide around to their side and attack their hind legs. It should also be noted that if he breathes fire, (SPOILER)just use Dragonrend and he'll stop immediately - I don't remember if it stops movement, too, but I know it stops their attacks and stuns them a little so I imagine it would stop them turning to have a shot at you (if you are attacking their hind legs).