In the end, Ghostpro's resolution was to use Draco's Law of True Destruction to deal with his portion of the grey rot and then move on. The moment he copied it, the drain was a shocking 25% per second, causing him to only have about 4 seconds to charge a rather powerful orb of destruction before firing it out into the grey rot.
The effect was obviously very good, with a huge portion of the grey rot directly vaporizing into nothingness. It might be resistant to many things, but nothing in this world was resistant to Destruction Energy apart from destruction-related beings.
Ghostpro checked the drops, which were enabled for him since he used Draco's law which also granted immunity rather than the raw element or energy itself. With a smile, he recharged his Divine Energy manually by stealing the mighty Shadowheart's law, then switched back to Draco's law.