stealthy marine own4ge
This is an elaboration/variation on MonsE's and tommyd's two-siege strategy.
Step one: divide your team in two groups; two marines, and the rest of them. Send "the rest" off to assault some alien-held area. Meanwhile, send your two stealth-ninja-marines towards one of the hives, preferably far from the area where "the rest" are making a fuss.
"The Rest" should attract a lot of attention - fire at all alien buildings/players, and if possible, take your time about it. They want to make a LOT of noise in hive sight. The stealth-ninja-marines, on the other hand, should make as little noise as possible; outfit them with jetpacks, and have them avoid conflict at all costs. BY NO MEANS should they fire at any target, unless they are spotted by an actual alien.
Two rooms away from the hive, have your marines build a turret factory. Yes, *two* rooms away. Remember, you're dealing with a pair of radii here - the factory doesn't have to be in range of the hive, just the siege turret itself. As soon as the factory is done, start upgrading it, and build a pair of turrets (one on either side) to defend it.
Pull "the rest" back from their assault and send them towards the stealth-ninja-marines; time it so they'll arrive as the siege is coming up, but not too long before.
Move your marines into the room adjacent to the hive, and build two more turrets, about 15 feet apart. Directly between them, place the siege. If you place two, have your marines build both of them ALMOST complete, and then bring them online at the same time - that way the aliens don't know what's coming until the hive is getting smashed.
Marine Stealth is a strategy with many uses; aliens rely heavily on hive sight, and that is their biggest weakness. As an alternative to sieging a hive, you can have a lone marine set up a phase gate, and then blitz the hive with a full-on assault.