Why do RPGs so readily allow us to create stupid builds? For example, that person has a stupid build.
What if they're not stupid, but creative? Why shouldn't the heavily armored mage who uses magic to stealth be unable to beat the game? This build would probably fail in a typical RPG. But it doesn't have to.
Add a system to measure the effectiveness of a build and adjust the game accordingly.
Think up your excuse plot as you like, but the general idea is to have the player fight a series of enemies which are representative of what they will encounter in the game. So they'll fight some armored melee, some flimsy casters, some archers who refuse to stay in place, and mixes of them, with varying strength. From these fights the game can determine approximately how well your build handles each of those enemies and adjusts their strength relative to each other. Now your build which is great against casters thanks to a quick attack from stealth does not need to be utterly useless against other melee. This will adjust both up and down, so that you'll still be stronger against certain types and weaker against others, but you won't ever fight an enemy which is trivial or impossible because of your build choices.
After the initial tests it would continue to adjust enemies for a bit longer, to ensure that if you got crushed by a certain archetype it was due to the build and not just ignorance of how to handle them. If you start rolling over a certain type of enemy, it will adjust accordingly.
For arbitrary numbers, let's imagine that against your build (and skill at playing it) a melee has difficulty 10, a mage difficulty 2, and an archer difficulty 6. Melee would be reduced in power, mages increased, and archers left about the same, so that when you venture out, melee might be at 7, mages at 4, and archers still at 6. You'll still see melee as harder than mages, but not so dramatically. This might be the difficulties at the "normal" setting, so dialing up to "hard" would make them 10, 7, and 9, while "easy" would be 4, 1, and 3. These examples aren't on any particular scale, just that bigger numbers are a harder fight.
Note that all this does is adjust the relative power of enemies. There is still an overall difficulty setting which can shift all of them up or down.
Coming tomorrow, an uninspiring, uncreative opinion piece on game difficulty.