The aether nymph is not terrible, it already has a high attack. The problem is they all have some rare ability that makes them so strong, say:
dark/death:
No high stats, but can kill off enemy creatures 1 by 1
earth: Petrifies over and over, not deadly but good. No high stats
purple: antimatter, can ruin your whole creature field! No high stats
fire: :Likewise, can kill all creatures with offensive rage. No high stats
gravity: Black hole in a bottle, can kill your entire game. No high stats
Air nymph: can do some decent damage in a few turns with gas. Attack is decent.
Light: Supporting ability for hope + high stats!
Time: High stats and a decent ability.
Water: Terrible nymph, the arctic squid is a better nymph than that thing. Already replied there.
Then comes aether. It miserably fails because the ability is so common. Nymphs ARE expensive, you don't get around that. If they get competition they lose easily. But what they offer should be good and fun. Immortality is lame, I say, resurrect the dead for aether nymph!