From what I saw of Imperiex (and given how bad most of that storyline seemed to be, I admit it isn't a huge amount) he could take care of the Annihilation Wave barring T & A being present or Thanos running off and getting the HOTI or something.
With the two near Galactus-level beings and Thanos with prep added it becomes a little murkier.
As for the zombies, I just laughed at the whole Galactus thing and I assumed that most characters in that universe were a hell of a lot dumber and less powerful. And Galactus just died, he wasn't zombified, so who knows what the virus would do to something like Imperiex.
Without T & A and no prep time for Thanos: Imperiex 10/10.
With T & A and Thanos having time to develop a plan for the Wave: Imperiex 5/10.
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Frankly, minus the whole zombies PIS thing, they don't have a chance. Imperiex Prime eclipses Galactus in power and unlike Big G, he doesn't run out of gas. And T&A are weak to the exact kind of energy Imperiex generates.
Worst case scenario, they break his armor, and the big bang nukes them all. Imperiex reforms after, game set, and match.
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