*possible spoiler alert*
If you are struggling with the warrior, there is a simple method of killing it.
Once the dragon spawns let it kill you so that you are outside of the barrier. Now, before the barrier you will see a yellow sign. Jump up onto the platform by the sign and then jump up again onto the rock. From here you should be able to shoot at the dragon in each of the 3 positions it takes whilst taking zero damage. Walk over to the ammo machine any time you need an ammo refill.
Hope this helps.