best to just throw it out at the start of the game, since it has the potential to one-hit-kill TIE fighters. before the TIE has even done anything, BLAM, removed from game. One less ship for your opponent to worry about, so you are doing him a huge favour actually XD. However, it is very very unreliable, so you will want to pair it up with the correct pilots, such as Wedge, Horton, and to an extent Garven. If you are not comfortable with early salvos, you could hold onto them and use them for range 3 kills, so your opponent will not be rolling 4 evade dices. But of course, the problem with holding onto them for too long is what if you were taken out while you still have torpedoes, and that would be a very big waste of 4 points. There is not much need to do a target lock + focus for the torpedoes, as compared to missiles, as it would not contribute too much to changing the results, so fire away!
I highly recommend aiming all your torpedoes at a single target at the very start of the game, either somebody who will mess your day up (like Vader or Howlrunner), or just somebody who happens to be in range (like Brian the academy pilot), then fire away until he is dead. Any remaining torpedoes, save them for your range 3 kills. If at any moment in time all your shields are down, use them immediately, never mind if they aren't at range 3 or you cant pull off a target lock + focus. Just use them immediately, before your ship go down with it. Better to spend it and waste it on 1 hit, than to die holding onto it. Also, the imperials are playing the numbers game most of the time, and reducing their numbers cut down their attack power nearly on an exponential level, so even taking down a academy TIE fighter with 1 hp left is worth it. Of course, if you could take down a named pilot, all the better, but as long as it can take down some TIEs, it is well worth the 4 points
Best used against Y-wings as they are less likely to mess up the attack dice results with a single evade dice (i have cases of Xwings flying through 2 cluster barrages and taking 0 damage). Combo it with marksmanship as much as possible. There should be no reason for you to fire the cluster missiles without marksmanship or focus, as they are only 3 attack dice worth. Yes, I'm aware that focus is only useable for one of the attacks, so here's what to do. If on your 1st roll you have 2 or more focus icons, use it. If 1 or less, wait for the second attack to spend it. If you miss both attacks, at least you could use the focus token for evading, so it isn't too bad either.
best used against X-wings as it is easier to confirm a 3-4 hit with them. Once again, abuse with focus for better effects, although marksmanship isn't really necessary here. Still, it is crucial that you use it together with a focus to ensure the hits do happen. It also helps you a lot for those range 3 attacks, especially since your main pewpew guns are worth a measly 2 attack dice
As you have less ships that can carry missiles, most likely you would only have about 2 missiles on your entire team, max. Instead of targeting everything on 1 Rebel ship, it is better to spread out your attacks and use it to soften targets, maybe even force them on the defensive (unless his name is Biggs, if Biggs is around remove him immediately. get everybody to target lock and focus and barrel him down. If you can remove him by the 2nd or 3rd turn, you will gain a very huge advantage). try to avoid using it on anybody with R2D2 until you have already dealt him some damage, so it will be more of a surprise and you might catch him unprepared or in a bad situation where he would not want to do green maneuvers. Also, if you used it on a fresh R2D2, you will only waste it knocking out his shields, which is can easily repair back. ideally you want to deal him some permanent damage. If your opponent has declared R2F2 as an action that turn, better save your missiles for the next turn or some other target. Your opponent is likely to be flying 3 ship formations, and it is highly unlikely for you to be able to one-hit or even two-hit Xwings and Ywings, so it is better to get all 3 to low hp and keep the stress on the player, with him not knowing who you would be aiming for on the next turn since everybody is now at 2 hp for eg.
Unless he is playing a 4 ship swarm, then it is a must that you aim everything at 1 target, preferbly that one named pilot that he has decided to field. This will severely ruin his swarmy game plans. either way, i see missiles more for softening targets than to actually outright take them out, best to leave that job to the standard 2-3 TIEs pewpew-ing it at range 1
EDITED, must be too tired, thanks for the nice catch. it's 2am here and I really should be sleeping right now.