Relax guys we just see no 8 Numbers...i bet the other Numbers have good design at least especially the no 2-4....the lower the number the stronger The Numbers is mark my word..this is like Espada type shit....
Why can't he win though? Killer is strong enough to fight him. On top of being more knowledgeable, He's better equipped to resist Apoo's abilities than Kid/Luffy/Zoro. He also has just as many reasons as Kid if not more to fight Apoo. He's the who suffered the most due to his betrayal after all.
Not saying it's going to happen but it's a possibility.
Where did I even say anything about the fight's difficulty? I merely said Killer is a good match for Apoo and that he may be the one to take him out since he's on the winning side. It doesn't mean he's going to have an easy time fighting Apoo.