User blog comment:Water Dragon Slayer/DREAM BATTLE CHRONICLES: FOURTH MATCH!/@comment-1393830-20130806042617/@comment-7418318-20130806084817

The problem (or in this case the opportunity) with Hades is, we don't know how much of his power came from the demon heart, even before he visibly activated it.

It might be something similar to how the Kyuubi gives Naruto incredible self-healing and a bigger chakra-pool even without Naruto actively drawing power from the biju.

With Hades it might be something slightly similar. Even without fully activating it he still might gain a magic power boost of unknown magnitude simply from being connect with it.

Now toss in that Hades is something around 125 years old and you have more than enough reason to make Hades as weak as you need him to be for your story, once he's lost his connection with the demon heart.