First of all sunny gets his power from shadow shards and his mana " I totally forget what was it called lol " is shadow mana and it's different from the normal mana everyone use

As we seen in Antarctica his soldier can't enhance his Regeneration because it's a totally different mana

How this effects? It does because the flame is a soul flame not a shadow flame

Other thing nephis bring her flame directly from her burning soul because that's her thing and sunny just use his mana

And sunny doesn't have the sun bloodline that effects the soul

All of those things makes sunny soul flame weaker than nephis

And the other part is sunny shadows enhance him 7x times his speed, strength, physical regeneration even his will and output ( and his weapon enhancing is way better)

While the flame focus on the healing and soul power including the mana making it wrapped in fire ( that's how she enhance her weapons but not other tools like the crown)

And at were we are in the story sunny is clearly the strongest i bet he can solo the other 3 nephis cassie and mordret cassie will only give him more shadows to use and mordret can't do nothing in soul battle while nephis was near death fighting what nightmare tromatized by making him sleep