Apparently you don't know too much about "nut bags". Prime example in the theater shooting, he planned ahead and had bombs made in his apartment. Just because a person is "crazy" does not make them stupid. In fact there have been some very successful people in the world who have suffered from mental illnesses. Do not underestimate will....
Yes, there are certainly some who would plan ahead...Timothy McVey of the Oklahoma bombing is a good example as is the Unibomber and it sounds like this freak in Rochester. And no, there is no one-size-fits-all solution.
Fact of the matter is that deaths caused by these "methodical" killers are a small percentage of the gun deaths total. Impulse-type killers are a much, much bigger problem. And if all we're left with is the methodical killer, then America is a much safer place.