People, the Supreme Deity isn't coming. Remember the curses? The Demon King put the curse on Elizabeth, and the Supreme Deity put the curse on Meliodas. In Chapter 311, the Sins find out Elizabeth's curse was not destroyed, because the Demon King was not dead. Meliodas' curse did not regenerate...
This is the perfect time for Diane to shine because she is now the weakest of the Sins. And since the lake is supplying unlimited power, theoretically means the more she dances, the higher her power can grow, with no upper limit. The power of the lake suits someone whose magic is of the land.
When Escanor got Sunshine back, the last panel showed him absolutely crushing Maels hand. Mael could very well be the better hand to hand fighter, but Escanor has more brute strength, which is going to be key in this fight, essentially
Mael uses the magical powers of Sunshine more/better than Escanor. Escanor uses the more physical attributes the grace provides him. Maels attacks would do nothing against the DK, as Inversion would cause them to heal him. Escanor is just straight out punching the DK with no magic behind it, so...
Rhitta was destroyed during the fight against the Original Demon. We see it cracking and falling to pieces after Escanor is kicked to oblivion by Death Drive
The Ruler/Inversion allows the DK to inverse MAGICAL ATTACKS/HEALING. During this fight between him and the Sins, we are not seeing...
When the Sins all find out Elizabeth's curse is still active and keeps regenerating, we see that Meliodas crude isn't. That would imply that, in order to end the holy war from 3000 years ago, the SD also surrendered her life and is no longer around. Maybe she inhabits something like Nerobasta...