Milim's sheer potential is crazy if you think about it.
Not only is Milim the only combatant in the entire series who didn't even receive a single powerup, when others like Zegion received tons, but she also never trained to get any stronger. She's still at the same level she was millennia ago, which shows how insanely high her base level actually is. Characters like Diablo, Zegion and Feldway all needed countless asspulls to get close to her level, while she is just built like that.
Now imagine if Milim actually took training seriously or if she got any powerups on her own. She would become absolutely unstoppable and only Rimuru could actually beat her at this point.