How useful is this card in mono-aether? Or is it better in duo?
It's generally more common to see it in a duo, but it can still be quite quite useful in a Mono. It's also strictly better in an upgraded deck due to Phase Spiders being semi-weak unupped; the only exception to this would be if you use Turquoise Nymphs instead, but very few people possess 4 or more of them so that's not extremely common. But since Phase Dragons and Immortals are completely invulnerable and the Spiders are rather frail, it's far more common for Mono Aether to forgo Parallel Universe in place of them.