Lahabrea having weakened himself with his body hopping isn't the worst explanation in the world, though it would require him to have lost something like 90% of his strength by doing this. It seems sketchy to me that someone as powerful as that (supposedly, anyhow) could lose so much of his power just by possessing a couple of humans.

Elidibus, though, doesn't make any sense to me at all. When talking to the emperor while wearing Zenos' body, he states that "with this adamant flesh", he would now have no trouble killing us. This implies that 1) Zenos' body does not weaken him and 2) he does not think that he could beat us without that body