Well, it's simple, I loved the heavy attack animation, so I did a build specifically for it.
I'd like to remark that there's no need to use an Umbral Forma, it's just for commodity. That being said, the other option is using a regular forma on Gladiator Might and Corrupt Charge/Vicious Frost, which could potentially limit other builds you'd want to do.
You could also spare a forma by swapping Primed Fever Strike for Virulent Scourge, in exchange of lower damage with higher status chance.
You can also swap Organ Shatter for Amalgam Organ Shatter for faster attacks, I don't because I like the build-up.

Funnily enough, this is just my [Sepfahn] Exodia Contagion build (with a Riven instead of Killing Blow), it just so happens to be good with heavy attacks