Here's what you need to know to One-Shot a Dragon:
Get your Alchemy up to 100 - you don't need to fill up the Alchemy Perk tree, though...just the ones that increase the potency of helpful benefits. This process takes a while...always keep in mind that the higher the value of the potions you create, the more the skill improves. If you make one potion worth 2000 septims, it'll increase your alchemy skill much quicker than if you made 10 potions that each are worth 100 septims in value.
In raising Alchemy, it's quality (value) over quantity (the number of potions you create) that'll raise the skill the fastest.
At the same time, go out and fill up every soul gem you can, regardless of size, or whether a petty soul is using up a Greater Soul Gem. You're going to be gunning to max out your Enchanting Skills as well...and again, it's not necessary to fill up your perk tree; just the ones that have to do with enchanting weapons and armor. When you're boosting your Enchanting skill, the size of the soul gem or the item is irrelevant. You could enchant 50 Iron Daggers each with a Petty Soul Gem, or 50 Daedric Warhammers with 50 Grand Soul Gems, and your enchanting skill will increase at the exact same rate. In this case, it's absolutely quantity over quality.
To get things to immensely strong levels, you'll have to dis-enchant any single piece of gear that raises your Alchemy Skill. You can then enchant a whole set (I use any Circlet, Necklace, Ring and Gloves) of alchemy gear using a filled Grand or Black Soul gem for each piece. Now you can make yourself Stronger Enchanting potions...which, when you drink them, raises the strength of your next enchantment.
Which means that you now make a whole NEW set of alchemy gear, stronger than the last one.
Which, again, means stronger potions.
You repeat this process over time until, in the Vanilla game with the Unofficial Patch, you'll have 4 pieces of Alchemy Enchanting equipment that each raise your Skill while you're wearing them to 32%. Your enchanting potions will max out around 32% stronger, and your Blacksmithing potions will improve your ability by about 130%.
Wait. We're not done. You'll need to create a set of blacksmithing gear at an Enchanting Station, as well....by using your maxed-out Enchantment potions to boost their stats!
Once you're done, you'll have a set of blacksmithing gear (I'd recommend a Blacksmith Apron, Gloves, Ring and Necklace)...that each raises your smithing ability by 32%. While you're wearing this set of Smithing gear, you'll now drink a "Fortify Smithing" potion that you've created using your maxed-out Alchemy gear. This potion raises your skill an additional 130% for 30 seconds, ON TOP OF the 130% boost that you get from the enchanted Blacksmithing clothing.
Long story short? If you've maxed out your One-Handed skill, you can create an Iron Dagger that does over 400 points damage per hit, not including a 28 point per hit Fire/Frost/Shock bonus. It'll kill just about anything in the game with three swipes.
This is a really rough outline, and I left a lot of specific details out...but that's the general idea. After installing Dragonborn, I've got a Dragonbone Sword that deals over 700 points of Damage on it's own, with another 200 of Chaos Damage.
You can go even more insane if you're using exploits on a console. You can get 5 pieces of both enchanting and Smithing armor, because of an unintended bug in the unpatched Vanilla game. I also didn't get into Dual Enchanting one set of gear that allows you to boost both Alchemy AND Smithing at the same time....
