Finish the Destruction Ritual Spell quest and buy the Lightning Storm Spellbook. If it lands and you can get the shot off, all the better: the spell works as a permanent stagger and the dragon won't be able to attack you until your magicka runs out. This is a good time to use the tankmage character build, and charge in with a greatsword to finish it off.
short of that, type "dragon slaying tutorials Skyrim" into YouTube.
I hate to say this, but do not do the above. Lightning storm does not stagger! Lightning storm is a great way to wipe out hordes of weaker enemies(civil war quests), but tougher enemies will just charge through the lightning beam and smack you in the face. Yes it does 100+ dmg/second, but a deathlord has 1000+ health so 1 deathlord will just walk right on through. In a fight several seconds is a lot longer than it seems, especially when every enemy takes several seconds AND you can't move away from them. Lightning Storm is the best master level destruction spell, but they are all pretty useless. What you want is dual cast thunderbolts. Does around 200 damage per cast, DOES stagger which will prevent the dragon from attacking, the lightning projectile is instant is it is good against airborne moving dragons, and you can still move around while casting it.
Having played many builds the mage with thunderbolts actually did the best against dragons hands down. With 0 cost spells you couldn't have a better character for taking down dragons. You could easily have many entire battles against dragons where you take 0 damage.
And yes 0 cost spells does work. Enchant of 100 + all 5 levels of the first enchanting perk are required, enchanting potions help.