I once heard of something that you can do where you enchant your armor so that destruction spells cost nothing. Is that possible? If so, how do you do it. Btw, I have my enchanting and smithing at 100

Just enchant whatever school you want with the 'Fortify X'. 4 of those at 25% with Grand Soul Gems will have you cast it for free.

But only the chest piece and necklace can have that enchantment and it only goes to 15%

Yes, a chest piece and a necklace, as well as a ring and helmet.

Make sure you're using Grand Soul Gems with Grand Souls in them.

And you pretty much need the Enchanting tree perked all the way in the middle.

easiest way (if you have dawnguard) is grab a bunch of empty Grand soul Gems and head for the Soul Cairn..

If i recall, in the Soul Cairn there are locations that if you put an empty grand-soul gem, it transforms it into a black Soul gem.. and since the Soul Fissures in the Soul Cairn will fill up one of your Soul Gems to the max that gem can hold, you can easily obtain enough full Black Soul gems for whatever enchanting you require...

You're trying to use the Cost Reduction + Regeneration effect which is also bugged to not give a proper amount of regen(it'll be -15% cost, +10% regen IIRC). You want the enchant that just reduces the cost of a school alone by 25%.

It also removes any challenge from the game as your character becomes pretty much indestructible, which is why I personally have never used it.

Drink a fortify enchanting potion/s right before to modify enchanting skill.

Not necessary if he has 100 enchant and the right perks. 5/5 Enchanter, Insightful Enchanter, and 100 skill is all that's needed.