Enemies/NPCs that are killed are immediately erased, so there’s no need to “revive” them. If they were a blue unit, then they aren’t erased when killed and are set to dead, so to revive them you would need UNCR which is added through a patch.
The exception to this is event battles, units killed in them are always set to dead, even if they are enemies/NPCs, which is why after the battles DISA is used to actually erase them.
So if you are not killing them in an event battle, they are already being erased, and so you don’t need a different unit ID and can just LOAD the same unit again. If it still doesn’t work, send a report with a savestate in the FEBuilder thread just before the boss is killed so we can investigate.