You wanted to change the level of a unit and I thought that was what you wanted it to do, give more xp to heroes.
"And what If I set the Doom Targets Allowed to include Hero?"
Well, it causes some weird gameplay to allow heroes to be targeted by the spell but not units of a certain level, heroes are after all supposed to be "stronger" than normal units.
So yeah, you can do that but I just feel like you're complicating things for yourself just because.. .. I don't know. Sometimes you've got to code around the problem as there are a few things not possible to do in Warcraft.