There's nothing that says you can't use heal block, only that you can't reverse healing.
So use heal block champs or champs like Warlock. It's a fairly easy path.
If you're not supposed to be able to reverse healing, it's bugged to hell.
I most definitely used both void and anti venom to reverse healing on a couple of the fights, the first fight and falcon to be exact.
But I wasn't able to reverse immortal hulk's healing, which was odd considering.
Lionheart node.
Says whenever u try to prevent or reverse healing on defender, u get 200% damage on u
It says whenever you reduce or reverse, not prevent. You can prevent healing.
Sentinel omega sentinel etc
Also you can use nullify immune champs too so have fun with broverseer