Ok, so being as I actually don't know what your play style is I will suggest my theories/practices of taking on a fabled Legendary Dragon from 3 play styles being as the following:
- Warrior Type - Heavy Armour 2h/1h & shield, bow offspec, restoration.
- Mage Type - Cloth Armour, 2h Spell Casting (Restoration, Destro and ALL other aspects of magic infused power skyrim allows.)
- Assassin Type - Light Armour, Bow main, 2x1h daggers, illusion & conjuration magic.
So firstly I will presume being at level 79 you will have the best of the best in items or close as all tactics should work the same but some maybe more gear dependent than others & all of the following are from what I find the best ways from my own experience.
Secondly I will also take into account you have potions upon your person for different situations & are currently wearing the Amulet that decreases shout time by 20% (Amulet of Talos).
Lastly I will take into mind you have a companion of sorts (I will just generalize the companion for opposite play-style for maximum damage output in situations)
So you're running along minding your own business as Dragonborn thinking about how cool it would be to Fus Ro Dah a goat off a mountain top or your wife that you've married because you love being a single hero (slipping in Riften if you catch my driften - Bad pun I know) and you hear the roar of a dragon, a big dragon you look up and find this huge beast throwing out Fire/Frost out of its mouth (Apply appropriate potion to what dragon actually is) you then take a deep breath and take a run towards its direction ready to slay the beast and take its soul , scale & bones to store and never look at again!
Warrior
Opener
Draw your weapons and Dragonrend the dragon. While he is dropping to the floor find yourself what I like to call a 'Nordic Advantage' (find yourself a set of trees, rocks, anything that can shield you in the worst case scenario that can give you a fair battle advantage as well) and use it purely as last resort. While this is happening your companion is waiting for the dragon to land or firing arrows at it & becomes bait. (I am sorry)
Fight Rotation
Now for the good stuff! Aim for its left back leg or right back leg, these are the points in which the breath / header (1 shot kill) and tail can't hit you, meaning an extended period of life as long as you do as I say. NEVER STAND STILL! Ever... You're just going to allow the dragon to rotate and beat the souls of its dead brothers out of you. Always rotate/move with the leg you're on to keep at its side for as long as you can. While this is happening you're currently beating it down with your weapon and repeating Dragonrend on cooldown (as much as you can keep it on the floor). Maintain the damage and good maneuverability and you will be ok and eventually it will die.
Worst Case Scenario
This will be pretty much the same thing to do with all classes... you mess up the movement the dragon turns around, what do you do? The dragon one shots your companion, what do you do? You run out of potions and have low stamina so you can't hit it as hard as you like, what do you do? You've no magic left to heal yourself as offheals, what do you do? Ran out of arrows so you can't kite him into the Nordic Advantage, what do you do? My answer is simple... RUN DRAGONBORN RUN!
Mage
Opener
Draw your spells (Lightning dualhand) and Dragonrend the dragon. While he is dropping to the floor find yourself what I like to call a 'Nordic Advantage' (find yourself a set of trees, rocks, anything that can shield you in the worst case scenario that can give you a fair battle advantage as well) and use it purely as last resort. While this is happening your companion is waiting for the dragon to land or firing arrows at it & becomes bait. (I am sorry)
Fight Rotation
Now for the good stuff! Aim for its left back leg or right back leg, these are the points in which the breath/header (1 shot kill) and tail can't hit you, meaning an extended period of life as long as you do as I say. NEVER STAND STILL! Ever... You're just going to allow the dragon to rotate and beat the souls of its dead brothers out of you always rotate / move with the leg you're on to keep at it's side for as long as you can. While this is happening you're currently beating it down with your spells and repeating Dragonrend on cooldown (as much as you can keep it on the floor). Maintain the damage and good maneuverability and you will be ok and eventually it will die.
Worst Case Scenario
This will be pretty much the same thing to do with all classes... you mess up the movement the dragon turns around, what do you do? The dragon one shots your companion, what do you do? You run out of potions and have low stamina so you can't hit it as hard as you like, what do you do? You've no magic left to heal yourself as offheals, what do you do? Ran out of arrows so you can't kite him into the Nordic Advantage, what do you do? My answer is simple... RUN DRAGONBORN RUN!
Assassin
Opener
Draw your Bow and Dragonrend the dragon. While he is dropping to the floor find yourself what I like to call a 'Nordic Advantage' (find yourself a set of trees, rocks, anything that can shield you in the worst case scenario that can give you a fair battle advantage as well) and use it purely as last resort. While this is happening your companion is waiting for the dragon to land or firing arrows at it & becomes bait. (I am sorry)
Fight Rotation
You're a badass, it's that simple, you stand toe to toe with anything and can outmatch anything at anything because well you're just simply that good... a lousy dragon will not mess you. So you go in for the kill, kite him with your bow (move around him and fire arrows) do this for around 10 arrows at his leg, run away into the Nordic Advantage, heal a little if you get caught and come back out rinse and repeat till he is dead he will be because well you're a bad ass.
Worst Case Scenario
This will be pretty much the same thing to do with all classes... you mess up the movement the dragon turns around, what do you do? The dragon one shots your companion, what do you do? You run out of potions and have low stamina so you can't hit it as hard as you like, what do you do? You've no magic left to heal yourself as offheals, what do you do? Ran out of arrows so you can't kite him into the Nordic Advantage, what do you do? My answer is simple... RUN DRAGONBORN RUN!
If you find yourself needing more distractions, natural human Skyrim instinct should kick in here, resto yourself & cast a spell to raise a minion.