Stun and tag
A tactic I use quite often against close enemies is 'stun and tag'. First you stun the enemy with a shot from your weapon, then you tag him with a plasma grenade while he's standing there recovering, conveniently motionless. If you're not actually face to face, you can make a brief forward motion as you throw, to send the grenade faster and lower. As usual, a potential benefit of tagging an enemy is that he might then run back among his pals with your fizzing gift.
A less serious benefit is that the tactic offers a whole lot of comical fun, particularly against Grunts and Jackals, who have such amusing reactions. I just love seeing those wretched Jackals running off squawking frantically with arms waving. Also note that if you've taken the trouble to tag a Grunt or Jackal on his feet, or at least low down, he's likely to shoot way up high when the grenade goes off. For even more fun, double-tag him on the feet. The first bang sends him up, and with any luck the second shoots him up even higher. See Launching Grunts and Jackals for related gleeful chat.
Stunning
The best stun effect against Elites, Jackals or Grunts, is obtained with a plasma ball, which takes down any shields too of course. Grunts can be stunned even with a normal plasma shot (ideally in da face), or a bullet from a pistol or AR. The same goes for Jackals if you can manage to hit their hide; but tagging a Jackal will need an accurate throw to the exposed part of his body if he's still got his shield. The stun effect for normal plasma fire won't be as pronounced as with a plasma ball, so you'll have less time to land your grenade.
Single action
When using a plasma ball to stun, you can be sure that the enemy is going to be well and truly dazed, right where he is. In this case I often send off the grenade almost instantaneously with the shot, particularly if I'm really close. As such, the whole thing actually feels more like a single action rather than one action after another. You almost have to feel sorry for the victim. One moment he's happily looking forward to giving you a good blasting, the next moment he's doused in green plasma and has a blue time bomb fizzing on his person. Sorry fella, but that's what you get for being Covenant scum!
Good situations
In the rockslide megabattle I use this tactic quite a lot in situations where lots of enemies are trying to funnel through a gap to get me. The Top pass retreat plan is a great example. You can listen out to hear what sort of enemy is about to come through, get prepared, pounce for a quick stun and tag, then duck back and wait for the bang. The tactic is also great for comical mopping up work against Grunts and Jackals. One after another, stun and tag; it's hilarious. Good practice too!