Can you also kill Players in ballgroups and other organized cross healing groups with this?
Ballgroups: No, they usually have near ~40k hp after warden's buff and probably picked the defense blue stars which you have no way around of, and I don't think any of them are a vampire for extra fire dmg for heavy attackers.
Heavy attacking a ball group is usually just hoping your attack hits the same time as someone else's burst, usually just siege. Or hoping a necro kamikazied into the ballgroup and applied off balance with scythe, then you could maybe oneshot the person with off balance. I also doubt some ballgroup members even wear majority heavy armor.
I think there was 1 ballgroup that had lesser hp around 34khp but can't remember the name, I think either Iron legion or some weird word like elnada with accents and you could onehit them
Organized cross healing groups: Most likely. Smaller groups have around 30-33k hp with vampirism stage 3 for undeath and elusive mist which is 13% fire dmg for the heavy attack, so they aren't really gonna be defensive as someone in a ballgroup against this build. If they have rallying cry's crit resists it could end up being a gamble.