2-0 vs Mr. X's anti-heal.
Pleiades is ridiculously good against ... well, everything, because of the tempo it creates. Easily best card of the archetype.
(Though, my deck isn't actually a Constellar deck... it's just that nobody's playing super-aggro so I can't use Machu Mech at all effectively. It has a lot of Constellar components, however...)