Who's the strongest Re Zero character Echidna could kill?

https://preview.redd.it/atjkkv2v5f0g1.jpg?width=1920&format=pjpg&auto=webp&s=cdcab38d9beb8d40b025371412710243347c5518

The author himself implies that Echidna is relatively weaker when compared to top tier characters in the series, in terms of 1 vs 1 straightforward combat.

But what if we consider war instead of battle?

For example, I'm confident she can kill Regulus, if she would have a will to do so and prep time, despite his insane authority.

Let me hear your opinions! Be respectful.