I just don't think power levels are much of a concern here, aside from maybe looking for someone with more power than your average human pleb, because if power was the issue, they've got pretty much anyone beat. I think it's more the mindset they're after, and then they will grant the power needed to accomplish the task. Also, if they are representing Eternity, it may be that as the embodiment of the universe and time, he can't actually alter his own self, and may need an outside agent to act as his hands/tool. Like, think of a surgeon who needs surgery. They have the skill and knowledge to perform the operation, but it is impossible for them to operate on themselves.
btw. apparently Eternity featured in the recent Dr Strange arc, and it's one where Strange, you know, made a deal with Mephisto, which is never good, so... But I'm wondering if that will tie in as well, it seems like it may play into the whole Mephisto thing they seem to be building towards in multiple books, but the presence of Eternity there is... suspicious, given the events in Loki and King Thor. But i have not read the arc myself, just some spoilers, so i don't know the details.