Sorry about that. The 12 elements in there are based on the online cardgame at elementsthegame.com (it's free to play and easy to register)
We're actually having a heated debate right now as to how the elements should be divided (paired up). You can read more of it here if you want:
Death element in that game is actually something like slow acting poisons, so the story counterpart of that would be the kind of magic assasins find useful.
For the Demon Lord, I imagine him as something completely non human and uses hell-spawn magic that can't be learned by anyone. It contains darkness and death, but is also mixed with things only the Demon Lord can comprehend. (I actually took the evil element from another online game, DragonFable).
However, being darkness and death, the Demon Lord's magic also has some weakness to Light. Hero was not able to land any of his ultimate killing blows (that would've killed Demon Lord then and there), so when presented with the opportunity, he settled for the next best thing by casting a very powerful Light spell which takes advantage of Demon Lord having Death and Darkness element. Even though Hero was not able to finish the Demon Lord for good, dividing his magic into tiny little pieces is more than enough to say it's a victory.
But the thing is, attacking usually leaves you open to attack yourself, so Demon Lord, as a last ditch attempt, attacks Hero with everything he's got. He employs a similarly powerful curse that restricts the Hero from using full 100% of his powers (possibly ever again). Now his legendary strength is divided into 6 parts, and worse, each 1/6th is now a sentient being that can think for itself.
Not only do these 6 waifus hate each other (insert sexist joke here), they kind of think "I'm the strongest element there is, and everyone else is just a scrub. That's why I was the Hero's favorite."
That's why, instead of taking the usual direction of an adventure novel (the waifus journey to destroy all 10,000 Demon Lord remnants), the female lead first has to convince the others to join her cause, and the only way this could ever be resolved is by brute force.
So the focus of the story would be female lead challenging the other waifus in battle to prove her strength, one by one enumeration style, like the girls in Busou Shoujo or the Generation of Miracles in Kuroko's Basketball. Maybe sometimes she wins, maybe one time she loses, maybe she works together with another waifu to bring down the top waifu, who knows. They all have their own motivations and desires, and just tolerate each other at best. The character development would come from them coming to terms with each other. Maybe they can even recreate the Hero one time when they all work together, (and if they're too fixated on arguing, maybe a second Demon Lord emerges from combined scraps)