He doesn't have to. The Flash will be able to kill Magneto a hundred times over before Magneto even realizes the fight has started much less has created a shield. Other than that there is always the speed drain and infinite mass punches.
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Carl - "No, the real point is: I don't give a damn."
Carl - "This line, here? Line of Death. You cross it, and your freedoms no longer exist. Um-kay? Have a good day."
The Flash is just a human? The last time I checked a human can't control kinetic energy, vibrate through soild matter, doesn't have superspeed preceptions, or can run at the speed of light.
While Wally doesn't going around killing people, there are several feats that prove the level of speed he is capable of and thus prove that he can easily defeat Magneto the pico-second the fight starts. Such as saving the entire population from a nuclear explosion by carrying 1-2 people (532,000 people to be exact) at once sending them 35 miles in and out away from the city (70 miles together) in 0.0001 seconds.
Not really Flash was once being held by Batman out of costume as Batman was in mid sentence Flash then appears ot of no where in costume basically saying if I didn't want you touching me you wouldn't.
There are also scans of him appearing in costume and out of costume in the same area because he was going so fast the image was still there and talking basically he was having a converstion with himself so the people would see that they weren't the same person.
So Flash can move faster than the human eye can folloow quite easily without gaining all this momentum first. He could run hit Magneto in the face before Mags even had a chance to put his shield up.
even if he did get up what is to keep Flash from simply vibrating through it.