Something like Shuma can't just be erased. Let's just say that the rules of the DC universe probably won't apply here. Especially since he is not a member of the DC universe, and whatever is going on between the Source and Darkseid and the Omega effect.
Also, If Crom, and the Celestials were not able to destroy Shuma, Darkseid sure isn't going to be able to either. At least, not as simply as just shooting the Omega Effect at him.
Plus, a being like Shuma isn't just powerful. It is pretty much the top rung of ALL magical beings in the Marvel Universe. It will have protection.
So, it won't be THAT easy.
Yer gonna have to get more creative.