The Shuma Gorath incidents need to be taken with a slight pinch of salt. There are several reasons for this. Firstly, when entities like Dormammu or Shuma manifest on this realm, they are weakened, of course. That is in addition to the fact that there is absolutely no evidence that a banishment or confinement requires one to be above Shuma (the fraction that is manifested) in power, the same reason why Dormammu gets banished by Strange, even though I reckon even his earthly avatar is above Strange.
The other main reason is that Shuma had been around for a long, long time before Crom sealed him off the first time. Crom would by situation already have something amounting to one-sided prep when his books bound Shuma to that mountain. That, once again, isn't the same as overpowering Shuma. Dimensional entities like Shuma traditionally have their anchor onto our realm as their only weakness, and exploiting that weakness doesn't put Crom anywhere near Shuma in power.
The last reason with regards to SHuma and Crom is a bit of a spinoff, but dimensional entities have proven to be extremely powerful in battle when their anchoring weakness is not exploited. I've mentioned Odin struggling to stalemate Dormammu's manifestation on the other thread, and this is the same Odin that has access to all the feats we already know of.
Besides Shuma, Crom has indirectly killed a bunch of Hyborians. It isn't enough to place him above Odin.