Kleiman also suggests (as he has before) the Mexican government should not target all drug traffickers equally and specifically target its resources against the most violent trafficking organization. While I see some weaknesses in this strategy, I do think it would be better than the current policy of the Mexican government.I certainly agree with the second paragraph. With regard to the first, I think it's worth pointing out that this isn't all that different from Mexico's present strategy. Mexico has definitely focused more resources on the Familia and the Zetas, because, they say, these gangs represent the greatest threat. That assessment may or may not be correct, but the logic is the same as what Kleiman is advocating.
One flaw I see in Kleiman's strategy is that he is solely focused on drug-related violence. I realize drug policy is the whole point of his article, but the violence in Mexico is not just caused by drug trafficking. Even if drugs magically disappeared, there would be significant criminal organizations taking advantage of Mexico's weak police and judicial institutions. For the Mexican government, they must focus beyond drugs on strengthening institutions and halting the influence of powerful criminal organizations that threaten the state and society.
And beyond that, Kleiman ignores the fact that the Mexican government hasn't proven capable of taking down the gangs it identifies as the most dangerous, because it has an utterly inefficient judicial system, jails that are completely uncontrolled, and police agencies that are corrupt and incompetent. As Boz mentions, the problem goes far beyond being able to identify the worst gangs (a strategy that is pulled from gang interventions in the US, where the context is very different); it's about building effective institutions first.
Update: Kleiman responds in comments.