NO, she really is not. Eliza is basically a unique highest tier vampire minus the combat proficiency, there are symbiotes which are not fully explored in the story so their full strength remain undisclosed, then there are entire clans like Rin's that exist in this story which consists of different elemental power users like Rin herself, and they have huge experience bonus over diana in everything, and then there are artifacts of substantial power that are owned by different guilds and around the world, put that in any highly experienced human adventurer's hand and suddenly they are demi-gods, last but not the least, the MC himself. And i'm not even going to include dwarves, fairies and other unexplored races and their WMDs and abilities.
She is not even remotely close to being above everyone else, unless you are talking about everyday npcs. But sure, whatever you say.
As for the rest of what you said, you asked about my problem with the fight with diana, not fight with gods and goddesses aund that i already answered, and diana is absolutely not a goddess. And if MC is only strong compared to regular human being then he sincerely would have died in the fight with diana instead of ending it in a draw, even more so with his fight with that giant mythological snake.
This makes absolutely no sense to me.