The Salve-Maker's Experiment skill will create an attack item from another kind of item in the inventory. The new attack item will be either the moderate damage item (such as Zeus' Wrath) or the major damage item (such as Parthenon's Wrath). How can I increase the rate at which major damage items are produced via Experiment?
I think experiment is based on the general value of the item. if you want the major damage items to be made, you need to shell out some ethers, hi-potions, x-potions, and things like that that cost a bit more money. I used to farm antidotes for items, since I'd only have to pay 10 pg per antidote, but I plan on moving on to remedy, then the potions and ethers to test out what kinds of items I get.
Still, I think It's a pretty safe bet that you'll be getting more major damage items if you use more expensive consumables for your experiments.