Twitch that finger
It was a long time before I got this thing into high gear. It used to feel as if the rate of fire had a lowish maximum, and that if my finger movements were faster than that, things stuttered a bit and felt unresponsive. In retrospect though, it must have just been my technique that was lacking, because by now the weapon has long since become a good friend. By using very light rapid twitches of the finger you can work up a tremendous rate of fire, making it potentially more devastating than a plasma rifle, quite apart from lasting a lot longer too. Rapid firing can wear down and kill an Elite almost before he knows what hit him. Along with its valuable overcharge feature and satisfying fast melee attack, this is a great little weapon and makes a superb combo with a pistol.
Overcharge shot, or plasma ball
A big plus is the overcharge shot or plasma ball, obtained by holding down the trigger briefly (less than you might think) before releasing. It wipes out Elite or Jackal shields in one go and very usefully stuns the enemy for a moment, giving you time to finish the job, possibly by closing in to give him a firm whacking. Of course, a headshot from a pistol is another way of finishing off, which is why the pistol goes so well with this weapon.
The homing aspect of the shot is also rather handy, though it seems pretty easy for an Elite to dodge it at anything beyond short range. They don't always bother trying though. I'd particularly recommend using thrust-firing when sending out the shot. Briefly run forwards as you charge up and release. I don't know that the shot will do more damage, but it'll certainly get there faster, which can make the difference between whether an Elite dodges it or not, or whether he has time to get off a shot before being stunned.
If you're ready to release a plasma ball but then change your mind about using it, you can effectively cancel it by hitting the 'Y' button to swap weapons.
The fast right hook is another big plus for this weapon. It's easily my favourite melee attack, and feels so satisfying. In fact, sometimes I'll pick up a plasma pistol just for doing a bit of whacking. The blow is actually delivered by your right fist after letting your left hand take the weapon, but it all happens so fast that you could be forgiven for never realizing it.
A plasma ball wipes out a large chunk of the charge supply (11 units), but that rarely matters much because plasma pistols are usually lying all over the place from the carnage you wreaked earlier. You did remember to wreak some carnage earlier didn't you? As for normal firing, the plasma pistol is highly economical for the amount of damage it inflicts, and lasts far longer than a plasma rifle. It gives 5 shots per unit of charge (while it's not getting low), compared to the plasma rifle's 2. If you're in a tight spot and running low on charge, it's probably best to limit yourself to normal firing, to minimize the danger of running out of charge entirely.
On the minus side, the shot speed is quite low, making it hard to hit a laterally moving target at anything above short range. The weapon is also quick to overheat, though not as bad as the plasma rifle.
Plasma against the Flood
Just in case you thought plasma weapons compare poorly with the assault rifle when used against the Flood, think again. A plasma pistol is far more effective than the AR; see the Flood onslaught challenge for discussion. You might prefer an AR for wiping out spores, but actually there's not that much advantage. Spores tend to be in groups, and if you pop one it usually causes a chain reaction that pops a few more. So, a few shots from a plasma weapon can wipe out a spore group quite easily.
Plasma pistol sniping
Although predominantly a short to medium range weapon, the plasma pistol actually has extremely long range with its normal mode of fire, namely about 154 metres, beating the pistol by a good 30 metres or so. The shots fall with distance but you don't get scatter like you do with the plasma rifle. A burst of shots will arc through the air in a perfect procession, allowing long-range precision fire. Unassisted aiming isn't particularly easy and takes a bit of finesse, but by pre-aiming with another weapon's zoomed view - ideally a 10x sniper rifle view though you could make do with a 2x view - you can gain sniper-like accuracy. Here's how it goes.
Aim high with the zoomed view to allow for the fall of the shot, then switch to the plasma pistol and send off a rapid burst of shots with quick twitches of the finger. As the plasma flies through the air, switch back to the zoomed view to watch, possibly moving to one side for a clearer view of the shots. If you got it right, the enemy gets assailed with a plasma barrage from out of the blue, which is extremely amusing, especially if he's remained unaware of you. Watching enemy reactions is part of the pleasure, and when the death is so far away that you hear no sound, it seems all the more comical somehow.
I call this mischievous technique 'plasma pistol sniping'. It certainly feels like sniping as you can be so lethally accurate from long range, particularly when using a 10x view. 10x pre-aiming is a trick more usually associated with long-range rocketing. As mentioned in the rocket launcher section, pre-aiming relies on the unrealistically perfect preservation of aim when you switch weapons, so I don't use it in serious play, but I certainly don't mind using it for a bit of fun. I suppose you might try explaining away this aim preservation by saying that MC has finely tuned muscle memory and control or something, perhaps aided by battle suit technology, but that's stretching it a bit!
I latched onto this technique while having cliffside hilltop fun in level 2, and it was pretty much like discovering a new mode of fire. Felt a bit like having a sniper rifle that fires plasma. Nice! You can have fun with it in many places, and there's a level-by-level guide to this elsewhere. Here are some tips on using the technique.
Ideally you want to be far enough away that enemies remain completely unaware of the incoming plasma. Not only is it a lot more amusing to see the enemy taken by surprise, it also leaves enemy movement quite predictable (maybe they're just standing around, maybe they're pacing). If you've already been detected, enemies tend to move around more, making targeting hard, and Jackals will shelter behind their shields. On the other hand, if you're undetected but within the enemy's earshot, they'll be alerted as soon as you open fire, which often causes movement and a miss.
It can be a challenge to kill an enemy with a single burst. Grunts are relatively easy, but with Elites it'll probably take a couple of bursts if you're playing on Heroic or higher.
Sometimes an enemy will be moving around but will stop when you score a hit. I'm mainly thinking of Elites there. This gives you a nice opportunity to pile in the plasma to finish him off. With this behaviour in mind, you could use the tactic of first sending in a spread of fire (perhaps by moving around as you shoot), hoping that at least one shot will hit and make him stop. Some enemies will run for cover instead though. In particular, Grunts tend to run around in a panic as you might expect. Courageous little critters eh?
Sometimes an enemy will be moving around, pausing for a while at certain points. Observe for a while to identify the pattern and decide on how you might get him. If he doesn't pause long in any one place, you can potentially fire off a plasma barrage towards his next pausing spot in advance (having taken aim last time he was there), so it piles in on him just after he gets there. Sneaky!
When sniping with a fairly level trajectory, you can use thrust-firing to raise the shot speed a little; perhaps by around 10%, though it's not easy to measure. This will also cause the plasma barrage to rake up the enemy's body, so you may want to aim a bit lower than normal to allow for this. I quite like using this technique when trying to get an enemy in one burst; sort of feels like you're trying to reach out and run him through. I'm not really sure this is any more effective than stationary firing, but I like the aggressive feel of it.
Incidentally, you can also fire plasma balls that are pre-aimed with a zoomed view. Unlike normal fire these seem to travel perfectly straight, but they also have shorter range of about 123 metres and they travel much slower. Here's a combination technique to try on an enemy. Send in a plasma ball, then quickly raise the aim slightly and send in a burst of normal shots so they pile in and finish him off while he's still stunned. Very neat!
Unaided long-range fire
At long range, aiming without the benefit of a zoomed view is harder but still feasible, and at least you'll have the satisfaction of knowing that you didn't use the pre-aiming trick. After plenty of practice I now use unaided long-range fire quite routinely in serious play, particularly against stationary targets such as Shade gunners. You might be surprised at what you can do. Obviously a sniper rifle or pistol would usually do the job of long-range killing more easily, but maybe you don't have those weapons. Maybe you're actually forbidding yourself from using them in the level, to increase the challenge of getting through (e.g. try tackling level 5 with only covie weapons and no vehicles).
Good eyesight is an advantage as your target may be barely visible, but you often get visual feedback such as a flickering shield, which helps you see whether you're on target. You can fine-tune your aim by moving yourself around, which allows more sensitive adjustment than using the aiming joystick. With moving targets it can be worth moving around slightly as you shoot, so that you send out a small spread of fire, hoping that a few shots will hit the target. In particularly you could move slightly from left to right (perhaps as you make a forward thrusting movement), to send out a small arc of fire.