Galactus is being overrated here. Even a fully fed Galactus cannot defeat Spectre, LT, or Thanos with IG no matter how much time he has, let alone only half a second. Spectre does not need the backing of the Presence to defeat Galactus he can do so on his own.
I fail to see why RKT is after Spectre, and Thanos with IG is after LT, but I suppose the OP can make it any order he/she wants.
In this scenario Galactus should stop at the first person with superspeed and is smart enough to use it to run far enough away from Galactus before he can react.
Spectre, under its own power, can defeat this Galactus based on what?
There is on panel proof that Oblivion is greater than the Chaos King. And Chaos King was stated to be equal to Eternity. Thus, going by the new material and canon, Oblivion should be greater than either Infinity or Eternity.