i think it is way too overpowered and should either only effect one row or it shouldn't effect airborne creatures. it just makes it a bit possible to defeat neptune like that without using either the pulverisor or explosion or masses of black holes.
it sounds like the ONLY reason you're saying that this card is OP is because you can't beat neptune. Don't you think his 12 shockwaves actually come first than inundation? The god has freakin 12 shockwaves and freezes which can combine to instant-kill any creature.
Also it sounds like you were frustrated while trying to defeat neptune by using RoL-hope deck. In that logic, are you going to argue that fire shield and unstable gas are also OP, since they kill off those RoLs?
In truth, inundation is one of the most useless cards. The only decks where there are more than 7 creature are fractal decks and creature spam (especially life). Every card is meant to be a counter to another strategy, and you can't argue that a card is OP simply because it easily counters another card or a group of cards.