The Plasma Pistol fires plasma bolts and if you hold down the fire button it will charge up and when you release it, it fires a charged plasma blast which homes in on your target. (Do not depend on the homing ability too much to hit your target- it is not that effective. But if you move your cross hair by a little bit, the homing blast will get it.)
If you tap the pistol fast, it fires just as fast as a plasma pistol, yet it does not overheat as fast. A charged blast takes away a shield. Make that an overshield, a Jackal's shield, an elite's shield, yours. If you hit a basic yellow/orange grunt, it will take it out. Two or three for red grunts. It's a powerful weapon. Take out an elite's shield and switch to a pistol to finish it off with a pistol shot to the head. Really useful. And the beauty is that you can find it everywhere.
The most important tip I can tell you here is the fact that a charged blast will not only take out an elite's shield, but stun it for a second. The key is to throw a plasma grenade right after it is stunned. This is practically an essential to know for Legendary games, where all types of elites excluding your basic blue jaw carry overshields, so sticking them would not be enough sometimes. It is a bit difficult at first, you might hit the wall behind the elite or throw it between his legs, but most people will get the hang of it in no time, and Legendary will become MUCH easier. When you become better at it, Pillar of Autumn will be a breeze.
Follow this tip very carefully and that pesky Golden Elite will find himself without a shield and a plasma grenade stuck to his face. This tactic is incredibly effective. I hope this will help you get through Legendary.