I am sure you will find better advices than mine, but I would say (my humble opinion of course):
-Keep support tokens mainly for defense (unless the marine has more than two) It is way better to ensure a marine´s survival than killing one genestealer.
-The red team is the worst. If you need to leave one out, that one should be it. Their abilities are very situational and even the "Full Auto" is not that useful as it appears to be: you will kill 1 GS almost for sure, but 2 GS only 25% of the time and 3 GS only 1 out of 9 tries. In a 12 marine game Leon is great covering the back of the others (because of increased range) and Valencio charging doors is good too, but in less-than-12 marine games, leave them out.
-When eliminating GS, remove first the ones with the movement symbol that is less frequent in the formation at that moment. That will help to lock the swarms in place when the next event card is revelaled (unless you prefer them to move of course)
-Normally it is better to loose one marine with an special ability from a team, than losing the last marine from other team (and all the order cards from that team) The exception would be of course Zael and probably Lorenzo and Calistarius: they are terribly useful and maybe losing them the last is better even if other teams need to be sacrified in the process.
-The green team can kill GS when rolling 4s in the die. Dont think too much about the special effect of killing 3 in one attack (as nice as it is of course), but consider the 4 as another skull when doing the chance calculation of attack success: the green team has the better shooting accuracy after the flamer.
-Blocking with Gideon Shield, using the force field of Calistarius, using Defensice Stance from the yellow team etc are life savers against the brood lords. However, against swarms with no special abilities, is usually better to try to kill them directly. Blocking them doesnt solve the problem, just postpone it.
-Lorenzo when defending must have the support order card in play and must be facing the swarm. With a couple of support tokens, he will be able to survive very easily against 3 or less GS AND probably kill them in the process.
-Dont use the teleportarium unless you really need to and you can do it safely in the first round after traveling to its location. The risk of losing marines or the certainty of using that many support tokens are normally not worth it.
-When using the move order card from purple team, and deciding if moving the event card to the bottom of the event pile or not, dont consider only the effect of the event card, but also the spawning and movement of genestealers. Usually the cards with "good" effects on the marines spawn way more GS (or move them to inconvenient positions), so sometimes could be better to keep a card with "bad" effect but "good" spawning.
That would be some of the things I found more useful to win. Please feel free to criticize them!
FOR THE EMPEROR AND THE GOLDEN THRONE!