The single important lesson: A combat skill requires two other skills to be really powerful.
- For One/Two Handed weapons, you need Smithing and Light/Heavy Armour skills to maximise combat efficiency. The skill by itself isn't very powerful. Unless you use Smithing to make your weapons stronger, and unless you use Light/Heavy Armour to increase your damage resistance... you won't get far.
That same applies to Destruction. Destruction requires Enchanting and Alchemy to be powerful. And BOY does it become powerful.
1. Enchanting.
With the right enchantment skill, all Destruction spells cost zero magicka. Yes... you read that correctly. ZERO MAGICKA. Please stop and grasp the profundity of that result. You can actually legitimately enchant Destruction to no longer require magicka AT ALL. It becomes an unlimited, indispensable, perpetual source of combat attacks. You can literally just keep casting any Destruction spell immediately after each other, for an infinite amount of time. It's insane.
2. Alchemy.
Alchemy increases the damage of Destruction 160%. To put that into perspective, the spell Incinerate will deal 90 damage (when perked). Combine that with the highest Alchemy potion, and that 90 damage becomes 234....
Lastly..... The Impact perk. ESSENTIAL.
I can't stress how important this perk is. When you dual cast a spell (which doesn't cost any magicka at all and can be done repeatedly for infinite), enemies stagger. ALL enemies. Even Dragons. What that means is that you completely hijack their usual movement and they are unable to move or escape as you bombard them with spell after spell as they helplessly flay about, unable to fly/run away. Their death is inevitable. And they didn't even get to TOUCH you. Seriously... It is almost TOO easy. I play on Master difficulty and can still take down an Elder dragon within 20 seconds using the Destruction (Enchanting/Alchemy) combo.
TL;DR
In conclusion, with three simple steps, you can make Destruction ludicrously powerful:
1. Enchanting to reduce the cost of spells to ZERO, making them infinitely available.
2. Alchemy to make the damage very high.
3. The Impact perk to override enemy movement, keeping them stationary while you bombard them with spells until they die.