I use a herald inside of a deathstar. For example, tzeentch herald on disc in a unit of screamers. 3++ re-rolling 1's isn't going anywhere. And that's without cursed earth.
Or a drone-star. Basically the grim will make any unit (even dogs) very tough to remove because of the (at minimum) 3++ that it provides.
A
FMC is a good idea to, especially a
LOC for the increased chance of saving a wound. Nurgle isn't great because they typically want to be in combat at some point, but I've even used it on a bloodthirster to great effect. Was the bloodthirster buffed by Be'Lakor? Sure but who's counting?
Just be careful to bring Fatey if you're going to use it on a big unit. Nothing worse than watching your deathstar evaporate because of a bad grim roll. That's why I particularly like using it on the plague drones and/or the screamers, because they can use cover saves to mitigate damage from a turn of a bad grim (provided they're not in combat of course)