Apocalypse smashes him, I doubt Omega could kill Apocalypse, not actully it wouldn't kill him, I mean to say hurt him enough (if it couldn't than it would be like Darkseid vs Doomsday). The problem is that you only think about Omega beams, what about if Apocalypse shoot at him first and hurt him, what if he crushed him first. It doesn't deppend only on Darkseid omega beams, Darkseid can be hurt to, and Apocalypse surelly can hurt him. And Apocalypse body when he wants, can be really hard to even hit.
Apocalypse is a mutant who possesses superhuman strength which he can augment by psionically drawing on outside energy sources. Apocalypse can alter the atomic structure of his body at will in order to change his form. He can even increase his size by taking on additional mass from a presumably extra-dimensional source. Through his ability to alter his form, Apocalypse can give himself virtually any superhuman physical power. Apocalypse's "costume" is actually part of his body, and he can psionically alter its appearance at will. He can levitate himself telekinetically.
Apocalypse has an extraordinarily long life span that has already lasted thousands of years. He can survive for weeks without food or water and can rapidly recover from injuries that would prove fatal to normal human beings. In the future, however, his physical form will eventually grow too aged and enfeebled to contain his vast superhuman energies. Hence, he will transfer his consciousness and powers into a succession of host bodies, abandoning each one when it too grows too old to contain his power.
are we talking about old darkesied or the one now. I don't think Batman could get beat as bad as the ass whoopin superman gave darksied. It wasn't even a challenge for supeman. Darkesied is no longer on par with thanos. The idea of thanos may have come from dakseid but darkseid is far from thanos equal.