Posted May 31st 2006, modified later
The basic idea
The level 5 fun Elite fireworks was about getting some oddly frozen Elites who you could then send shooting up into the sky like fireworks, leaving behind a glorious pink trace of needle explosions. Well, here's all about doing something similar with Hunters in level 4, but this is easier to set up.
You'll end up with a pair of frozen Hunters at a launch area you've chosen. To launch a Hunter you'll blast the ground under his feet with multiple rockets, fire multiple needler clips into him if you want an explosive pink trace, maybe tag him with a few plasma grenades for good measure, then (typically) back off in the right direction to see him suddenly shoot up, potentially going so high that he becomes a tiny dot, even in a 2x view.
Here's a step-by-step plan for creating a good save. Although you can use any difficulty level, I advise against using Easy as Hunters are so hesitant about chasing you, making it slow and awkward to lead them to the launch area. In this plan the launch area will be by the ammo outside the path entrance to the security substation. Other launch areas are possible but you can read about variations later.
(1) Fight to the security substation and collect needlers
After landing (you don't need to stay for the battle if you don't want), head off left and fight through to the security substation to release the door to the main facility. As you go, collect maybe six full needlers into a temporary collection point somewhere, e.g. where the circle of Jackals was. Also fetch the rocket launcher and add it to the pile. You may want a pistol too, for watching your fireworks.
(2) Fight to the Hunter chamber
Proceed towards the 'Hunter chamber' in the main facility (two floors down), killing all enemies you meet on the way. I suggest you save before arriving at the chamber, so that if anything doesn't go well enough in the remaining work, you'll always be able to return to that point by ejecting the disc and reloading.
(3) Move the Hunters to the launch area
Some Jackals are in and around the chamber, but importantly, don't kill them all (explanation later: see Don't kill all the Jackals). Lead a first Hunter up out of the facility to the launch area: the ammo area outside the path entrance to the security substation. Head back to the chamber, leaving the Hunter behind. On the way there's a loading point down a ramp from the facility's entrance level. If you were to now return to your Hunter, you'd find him frozen; some odd consequence of this loading point business. No need to go back yet though. Continue on and bring the other Hunter out. Once he's in position (preferably a good few metres away from the first), go back and get that loading point on the ramp. Returning to the Hunters, you'll find them both frozen, ready for business.
(4) Finish off
Prepare a needler collection a short way away from the Hunters, but not so close that it might get blown away when you rocket them. Get armed with the rocket launcher. Make sure you're fully stocked with grenades, and preferably with an undamaged overshield (just in case you get too close to an explosion you set off). Get in a Warthog and get a checkpoint at the nearby arch. Save and quit, to complete your save.
Having your fun
Now you've set things up, here's some talk about the subsequent fun. This has much in common with what what I said for Elite fireworks, but there are a few differences.
As with the Elites, you can get a strange 'anti-aim effect' that seems to repel your aim away from the body. With a healthy frozen Hunter, things initially seem ok, but after you blast the ground beneath him with a rocket, you may get the anti-aim effect if the Hunter isn't quite dead yet (reticles of weapons other than the rocket launcher go red while he's still alive). The effect can sometimes be so strong as to make it frustratingly hard to aim a second rocket anywhere near. However, once you manage to blast him again and kill him, the effect disappears. Alternatively you could make it go away by whacking the Hunter until he's dead. See Avoiding the anti-aim effect for a tip on avoiding the effect entirely.
Incidentally, you could do some needling before using the rocket launcher if you want. The needles initially home in. As you continue, you'll find that the anti-aim effect starts up; but although this diverts your aim, it doesn't stop the needles homing in, and they'll swerve quite dramatically to reach the target. As you needle, the Hunter suffers occasional explosions, and after a while these will kill him. Once he's dead, needles will no longer home in.
Those frozen Elites were subject to disappearance after a certain number of needler clips. I haven't seen Hunters disappear like that, but I have observed the following. After about 25 clips, I could no longer see needles exiting the needler, even though I could feel them. Also, the sea lost its texture, becoming a uniform block of colour. I believe this was all due to the game reaching the limits of what it could handle.
To launch a Hunter after blasting the ground underneath him with rockets and injecting him with lots of needles, you need to cross a 'release line' (one of those mysterious things which I talked about in the Elite fireworks article). For Hunters near the ammo area I mentioned, backing off towards the sea will do the job as there's a release line out that way (I'll add a picture of it later, when I add pics for the whole article).
You can potentially launch the Hunters together, giving 'synchronized fireworks'. For the best effect, try to apply the same effects to each Hunter; same rocket boost, same number of needler clips, same number of plasma grenades. This should give the most synchronized look. When the grenades go off though, your fireworks may shoot off in different directions to some extent.
Aiming your firework
You can aim a firework according to where you place your rocket blasts. Put them between his feet and he'll be going practically straight up. Put them off to one side and he'll fly off diagonally. You can potentially fire him into a cliff to make him bounce off. Ouch!
More rockets means more speed. Give him all you've got and he can be up in the air for almost half a minute!
If you've also tagged your Hunter with plasma grenades, they'll go off together and give him a sudden boost in some direction; maybe shooting him up higher, and maybe sending him off at an angle. For the most extreme effects, concentrate the grenades in one spot on his body.
Clay-pigeon shooting, cyborg style
See if you can shoot a Hunter in flight. The pistol is the obvious weapon for this, but you can also have fun trying to rocket him. It's not easy with the launcher as rockets are so slow; but at least the target is quite large. If you've not sent him up far, see if you can get him at the top of his trajectory. Otherwise, maybe wait until he's nearing the ground, and fire almost horizontally at where you reckon he's going to be.
When you've emptied a needler but haven't finished needling yet, preferably pick up a new one (by swapping the empty one for another type of weapon, then swapping that for a new needler) rather than walking over another needler with your empty one. If you do the latter, it's wasteful because you don't get the needles in its current clip; they disappear with the needler!
Underwater needler explosions
If you send a Hunter flying out to sea, you can potentially drive after him and reach him underwater before the needler explosions stop, to see what they look and sound like.
An incidental but enjoyable thing you can do with a frozen Hunter is to send shots of various types pinging off his shield. In particular, see if you can get plasma shots to bounce off and whistle past your ears. See the separate fun item Pinging shots off a frozen Hunter for more on this. With this activity in mind, you may like to take the time to place plasma weapons and others with your needler collection when setting up.
Set-up variations and tips
In the save I described, the launch area was the ammo area outside the path entrance to the security substation. I suggested this for a couple of reasons. It's not far from the main facility, it's conveniently close to the needlers you assembled, it's conveniently close to a checkpoint you can use for the start of the save (namely one you get at the arch), and there's a release line fairly close.
Later you may like to try other areas, perhaps because you like the look of the scenery somewhere. Some places may not be suitable though, because there's no convenient release line. I may add pictures of the release lines later to help, but in the meanwhile you'll just have to experiment. If you find anywhere you think is particularly good for some reason, I'll be happy to hear about it if you care to drop me an email.
A base save for experimenting
If you're interested in trying out different launch areas, it'll be useful to have a 'base save' to use, in the interests of efficiency. Here's something appropriate.
Follow pretty much the same instructions for a normal save up until you reach the Hunter chamber, except (1) eliminate all enemies around the island, so everywhere is clear for use as a launch area, and (2) have a rocket launcher with you as you approach the chamber (I suggest you make it a secondary, with something else for warding off Jackals). One by one, lead the Hunters out and up to the next floor. Briefly head up to get a checkpoint. Save and quit, to complete the base save.
In this save you've already extracted the Hunters from their chamber, so you won't be needing to do that again and again each time you try a new launch area. That saves labour. The reason it's good to be carrying the rocket launcher is to minimize the time it takes to decide out whether a particular spot is good for launching. Once you've got a Hunter there, you can go back to get the loading point at the main facility, then return to your newly frozen Hunter for experimentation (forget the other Hunter for now). Rocket him a few times and see what it takes to set him off. Keep reverting and trying again until you've decided whether it's a good launch spot or not.
If it's a good spot, you could then create a 'proper' save in which both Hunters have been led there and you've got your needler collection nearby. You could use a copy of your base save for this, to save time.
Don't kill all the Jackals
Why did I say to avoid killing all the Jackals in the Hunter chamber? If you kill them all, I believe you'll get the following checkpoint annoyance in your save. As soon as you kill the second Hunter (probably while rocketing), you've killed all the enemies that were in that chamber, and this triggers a checkpoint. As such, you won't be able to simply revert to get back to the start of your save.
Leading and positioning your Hunters
For general advice on efficient Hunter leading, see here. As long as you're on Normal or higher, you should be able to have him running after you practically the whole way from the chamber, with barely a swipe from him (maybe just a few to get him through a couple of doorways). On Easy though, Hunters are hesitant about chasing after you for a swipe, making it slow and awkward to lead them around.
When a Hunter is at your launch area, here's how you can leave him standing there (as long as you haven't damaged him so much that he's in rage mode). Face to face, let him take one last swipe at you. Sidestep and run past as he starts swinging. By the time he turns around, you're far enough away that he won't run after you. He'll fire a shot or two, but that's no great worry. You can now head off back to the main facility; he'll stay put.
If you want your Hunter to end up facing a particular way (e.g. out to sea), just make sure you run off that way so he faces you, and don't alter course until you're so far off that he's lost track of you.
When you bring along your second Hunter, you'll probably want to position him a good few metres away from the first, so you'll be able to launch them individually. However, you do also have the opportunity of getting them very close together. In that case, rocketing will affect both at once, and they're likely to always launch together (synchronized fireworks). Could look good!
Avoiding the anti-aim effect
If you want to make your save as nice as possible, here's a tip that should let you avoid the unpleasant anti-aim effect entirely.
While leading each Hunter into position, whack him enough times that a single rocket would kill him, but not so many times that he enters rage mode (you don't want rage mode because it would be hard to leave him standing at the launch spot; he'd probably keep chasing you). Later when you blast him with a first rocket, the rocket kills him rather than merely taking him to near-death. The anti-aim effect therefore gets no chance to kick in. Seems to work.
So how many times should you actually whack him? That depends on the difficulty level and his toughness (he may be strong or weak). I suggest you do things like this. After getting a checkpoint somewhere while leading the Hunter along outdoors, do an experiment. See how many whacks it takes to kill him. Revert, and whack him enough times to take him to within five whacks of death. I think that should do; he's still a few whacks away from rage mode, but a single rocket should kill him.