Thanos relatively easily, neither Parallax nor Ion was beyond physical harm, both their powers comes from:
1. Central Battery of GL and
2. Left Over Energies from COIE.
Both the energies can be absorbed, given Thanos inherent ability to absorb energy and a power gem to augment it to unfathomable limit i see no reason why he shouldnt be able to do that. Then we have once they use their power they both do lose their powers. Zero Hour Parallax lost all his powers while re-constructing DC timeline and a fight with Spectre, Ion, whose source of power is exactly same lost his powers while i re-ignited the central batter with all his powers.
Furthermore, feat wise Thanos has far better feats than either of them anyways. Hyperbole wise, Thanos has the power of infinity being, only second to LT. I certainly dont see why or how an argument can be made that Parallax or Ion can beat Thanos, unless one, wrongly believe, IG on full power can actually be taken away from you unless the user wants (Something Mephisto tried against Thanos to no avail).