A good place to get the "Negative Ramos!" achievement early is in the "American History Museum" main mission on the Story difficulty. During the "Reach the archive room" objective, a medic with a purple health bar will spawn. Medics have a "+" sign above their health bar. They carry a defibrillator on their backs. While a medic revives a dead enemy, shoot his backpack to make the defibrillator explode, thus shocking the medic. It only works while the medic is in the process of reviving an enemy. You will be fighting a lot of medics throughout the game, and this achievement will most likely come naturally.