Here is what I have found in Multi Star Trip (feel free to correct me if there is any inaccuracy) :

• For H2O planetaries (Geolyte and Oleana), the rule is the same : push as much as you can to give big doses of burnt Meteos to the opponents, one by one.

• Both following branches are fine to follow. The best is to ensure you avoid Mechatropoloids (Grannest and Megadom). As both are easy to play, they also can doom you in Brutally Hard. Moreover, doing a Clear Screen in Megadom is near impossible if you do not do it properly, and you would have to succeed the mission in Bubblies (Starrii, Vubble and Florias) to possibly have a chance to face True Meteo.

• A special rule is given to all top groups and Bubblies in Brutally Hard : the amount of Meteos you send to the opponents is ridiculously high, compared to what they send you. For instance, even if you send around 30 of them, you can fill a whole screen of one opponent. So make sure to carefully ensure you you have done the Mission before.

• For Earth lovers (Forte, Anasaze and Cavious), both Forte and Anasaze work the same : once you get all of the Meteos, do a vertical ignition to send everything back to space. For Cavious, if you can, go both ways and center to do one full block. Also, make sure the Touch Screen lights when you do a Screen Clear, as caeonosphere stated.

• Bubblies and Dimensionals (Thirnova, Gravitas and Gigagush) work the same. Pile everything you can of the needed kind. You need a success if you go for Bubblies.

• Hot and Chillies (Freaze and Dawndus) work the same too, but Dawndus gives more time to chain the Meteos. For Freaze, chain as much as you can, as it is a 7-column long planet. You can permit yourself to do a 3 and a 4 ignition, then join them.

• For Clean Air Club (Yooj, Brabbit and Bavoom), you often will be given the worst planet to do the mission (send 75 Meteos in one go), which is Yooj. Bavoom and Brabbit have gimmicks to achieve the mission, so if you do not wish to face the 3 with Yooj, just annihilate yourself to be given a chance to do it with another planet. You can start with a 3 or 4-column wide ignition. For Brabbit, push the accelerator once all 9 columns are under an ignition. And chain as much as needed. For Bavoom, it is almost the same, but you need to stack a lot more. But for Yooj, you need to prepare two 5-column wide ignitions that you join with the remaining column. Then stack as much as you can before launching all the ignitions. Normally, that should be enough to send 75 in one go.

• The Rare Cultures (Globin, Lastar and Suburbion) are easier to eliminate due to the special rule, same for Hevendor Realm and True Meteo.

And I think that is all I have found.