If you can manage your positioning, you should find the fight against the Guardian Dragon a bit of a doddle. We'll walk you through all the steps you need to take to emerge from this end-game fight unscathed.
How to kill the Guardian Dragon
The most important thing to know about the fight against the Guardian Dragon is that you can simplify things considerably by hanging around the creature's rear legs. That'll keep you away from the danger of its fire-breathing attack, as well as the swiping attack from its mouth. If you see the beast lift a leg, move away just a little to avoid the stomp, then get straight back in to dish out your own damage.
If you see the dragon lift off into the air and hover above you, make sure you get away from the stomp that's coming your way. If it flies off to another side of the battle area, get as much distance as you can between the pair of you - the long-range breath of fire will really mess you up otherwise.
When you head back in to carry on fighting the creature, just watch out for a couple of close-range attacks that the dragon makes use of. As you try to get back into position, it may breathe fire at you or bite you hard. Dodge to one side to one side to avoid the fire breath, and dodge forwards or backwards to avoid the swipe. If you're really close to getting back into position at the creature's hind legs, roll forwards. You'll need to use your own judgement about whether to play it safe here or not.
Once you've killed the Guardian Dragon, make your way through the next door along, then go across the bridge. Head on down the hall, and make sure you grab the Dragon Scale that's just to the right of the door. Enter the lift to get up to the next floor above, then go through the door on your right and down the hall. This will lead you into Dragon Aerie.