The plasma rifle has higher shot speed than the plasma pistol but seems quicker to overheat. It also seems less damaging than a rapidly fired plasma pistol. Nevertheless, it's a powerful weapon for the close-up stuff, especially when it comes to those nasty Elites. It can reach out to medium range, but the slowness of the shot speed makes it hard to hit an enemy running across across your line of sight. In that situation you need to lead the target, i.e. aim ahead of them. Because this is primarily a short range weapon, I'd usually complement it with a pistol, which is superb for medium and long range work.
The technique of thrust-firing is generally recommended for this weapon, except that it's not too important when facing Grunts or Jackals close up, as they're so soft. When using it at medium range, I like to think of the plasma beam reaching out like a long jousting stick. Against moving Jackals, you'll want to time things so you skewer them with the beam just when the flesh is least shielded. You'll need to make allowances for the low shot speed to get this right.
Rapid short pulses
Another tactic is to use rapid short pulses to avoid overheating (or to delay it for longer). If you get this right, short pulses into the head of an Elite will keep him more or less stunned, and rapidly take his shield down.
Rush and whack
An alternative tactic to short pulses is to deliberately keep the trigger down as you rush an Elite. Depending on your accuracy, there's a good chance he'll be dead before the weapon overheats. If not, then you should hopefully at least have got in close enough to finish him off with a melee attack. But whilst rushing in, it's important to keep him stunned with firing to the head. Otherwise he might get hold of his senses, and all of a sudden you find yourself with an overheated weapon while still too far away to whack him. That's bad.
Plasma rifle sniping
Can you do 'plasma rifle sniping' like you'd do plasma pistol sniping? Well, yes but it's nowhere near as good. Firstly, the rifle has much shorter range of around 95 metres, so enemies will probably be alerted to your presence. As such, they're likely to move around more and you won't have the amusement of seeing them taken by surprise. Secondly, its accuracy is not as good. When you fire a burst, shots are increasingly likely to fly wide, reminiscent of the assault rifle. Thirdly, it's much less economical with its charge compared to the plasma pistol. Together with the lower accuracy, this means you won't be able to get as many enemies before needing a new weapon. Fourthly, the higher shot speed means you have less time to switch back to a zoomed view to enjoy seeing the effects of your barrage.
There's one other difference with plasma rifle sniping. Your shots don't fall as much. If laws of physics are being respected, this is something you'd expect due to the higher shot speed. They have less time to fall, over a given distance.