Originally Posted by
polyphonica
They did kind of address this in one of the optional dialog sequences with Emet-Selch. Lahabrea was weakening himself by constantly moving between bodies, which is something that Emet-Selch tried very much to avoid. Igeyohrm (like Nabriales) was also one of the sundered Ascians rather than being one of the original three. Emet-Selch also identified himself as a powerful sorcerer, so it may be that he always had more raw power than Lahabrea did. We also see, in the final fight, that Emet-Selch appeared to be drawing power from the faith of other Ascians to summon himself to his final form. I take it that, having thus tested us so far, Emet-Selch realized he needed to bring it all to bear in that fight.
For Elidibus, we can infer that he was also weakened by occupying Zenos' body, but it also may be that as the Emissary, his skill was never necessarily strength to start with. If/when he reveals his final form I assume he'll still put up a good fight anyway, but that his current approach seems to be to pit us against other WoLs is perhaps telling.