Typically I make the hardest mode first as the "original" difficulty, and then edit it to make the easier modes. If I do it the other way around, I end up accidentally making some parts of the game nearly impossible.
And yeah, "enemy attacks have a chance to pierce defense" or "enemy attacks pierce 50% of defense" would be better. You want defense to do SOMETHING. Otherwise why does it exist? I'm also not crazy about double HP, since making fights longer doesn't usually make them much harder in most games, but maybe it works well for you. I'd have to play both modes to be sure.