Hecatia. When going full power, she will not even play by the spellcard rules, as she is above them. She is also somebody who is pretty much in charged of hell.
Suika, Yukari, and Yuuka are also pretty powerful, but none of the three would stand a chance against the Yama if she was going full power.
Fun Fact - Eirin is way more powerful than Kaguya and Mokou. Eirin wasn't going full power in IN because she was trying to stall you, rather than killing you.
Fun Fact 2 - Flandre may be able to destroy anything, but she can't destroy things that are indestructible, immortal, or undead. One other thing to remember is that she IS a vampire, with the same weaknesses as Remilia. All you would really need to do is bring sunlight upon her, use water against her, etc.
Fun Fact 2.5 - Yuyuko suffers the same fate as Flandre, being unable to bring indestructible things to death.