Here's how the fight goes. Doom distracts Sinister and Apocalypse while Mags raises his shields and concentrates. After a few minutes, Mags opens a wormhole and sucks Sinister and Apocalypse onto one of Saturn's moons. He then reverses the charge in the protons and electrons of half of Apocalypse's atoms, effectively converting them to antimatter. Pocy then annihilates, taking the entire moon and Sinister with him.
How much prep is involved? Because with prep Apocalypse was a plot device away from turning himself into a Cellestial level being in The Twelve, and he and Sinister likely have the genetic templates of all the mutants that were involved in that, particularly X-Man and Living Monolith. He's unpopular (here), but he's not exactly weak or incapable.