Prep means Mags' shields will be up before the fight so speed blitzing and whatnot will not beat him. His shields have withstood everything from Galactus to Thor. Also, Atom isn't going to out manipulate Magneto in terms of taking out his shields. Magneto can draw from a wide range of energies and is way more skilled with the use of his powers. No way Atom simply takes his shields down.
I prob need to stop arguing tho since I'm at work and have stuff to do lol. I don't really have any more to add and I'm ok for judges to make a choice.
One more thing lol, the New 52 Atom is held together by his skin because his body's atoms are constantly splitting and reforming, which is where he gets the massive amounts of energy he has. The normal Atom before the DC reboot was an army officer who bonded with the Dilustel so that the metal covered his body, which means that he is still human underneath the metal and is vulnerable to Mags' attacks. At least that's how I see it. If Atom gets his New 52 form, then I dont see how he would get his past feats in this competition because:
In the new reality, Nathaniel Adam has only been Captain Atom for a few months and is still exploring his abilities, constantly learning new facets to them such as his ability to perceive wireless transmissions from cell phones and computers