-How did Danzo beat Shisui So easily?-

[NineSNS]

The element of surprise compensates for a big difference in power. Perhaps Shisui's only mistake: respecting Danzo.

 

[Sauce]

Shisui obviously didñ't know about Danzo having a sharingan, Danzo
appeared out of nowhere with Izanagi.

Shisui let his guard down too easily.

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I agree with this.