I've been thinking a bit, why do the aliens always have to lose, what are the flaws in their tactics. I hope they won't read this, otherwise we might have a problem in the future.
1) their greatest tactical mistake is simply not attacking all at once. Blitzkrieg would be an appropriate tactics to get Earth on knees, but not, they must send weak forces first, for us to learn from. A bit of well hidden spying on us with their superior technology, and then a massive offensive would do the job.
All right, let's assume, that aliens doesn't exactly have much military forces in this corner of galaxy, so it takes them a while to summon more spaceships and do some greater attack, while a temporary tactical withdrawal is considered as a sign of weakness among them.
2) so why don't they attack simultaneously on several places at once? X-Com, or Phalanx has only limited amount of people, which has to heal all the time, and they couldn't handle several missions all around the Earth. Or aliens could simply attack in greater groups, than 6 of them.
3) obviously, one of great aliens' mistakes is leaving their stuff behind, so local gibbering monkeys can tinker with it. Every alien should have a dead man's bomb (or dead alien's bomb, but it would make men dead too), which would explode and destroy the body, when a life signs goes off. This is, what good old Predator had, and it gave Arnold Schwarznegger a hard time. They would, of course, have to maintain a greater gaps between them. Some kind of autodestructive mechanism should be included in as much of their stuff as possible. Guns and ammo, while detecting human DNA on them, should blow up, and when not detecting life signs of it's registered owner nearby, should blow up as well. An UFO shot down should explode massively, as soon as an airship with a Phalanx squad gets close enough to be wiped out with the blast. Earthlings would thus get known with their technology only very shallowly. Of course, if a big smoking hole in the middle of a chest can be called 'shallow'.
4) but aliens should learn from humans as well. After all, chosen members of human race, properly telepathically controlled, chirurgically and genetically adjusted, would be a very good contribution to the forces of aliens. Tamans, Shevaars, Ortnoks, Humans, that is, what the citizens of next planet under invasion, would fight with.
Aliens should assimilate knowledge and ideas of every race they conquer, to become really strong, and prepared for everything. Their armies could do wonders with a sniper rifles, of course, in form of superior alien technology, which would enhance this human invention.
And what about Nikola Tesla? His fearsome inventions, rejected and misunderstood by foolish authorities, would unfold their true potential in tentacles of alien scientists. Aliens, at least those from X-Com 1, would also love a psychedelic music genres of this time, in combination with their colourful entertainment chambers on their bases.
5) aliens should also get rid of their shyness. Yes, they're on a planet which is not theirs, and they do a mess there, so they have a good reason to be ashamed. But if they want to feel at home here any time soon, they should stop hiding into a building every time they see (and shoot into guts) a human. A strategic position, crouch, autofire, combined with agressive progress, would be much more appropriate. I'm sure they can abduct a several good human psychoanalytics, who can let them lie in a comfortable slime cocoon, and talk over this problem together.
Of course, there are honorable exceptions. At Rome, there was an alien attack. My squad captured both bridges over a river (Tibera, probably) and prepared to proceed further. Aliens of course opposed, but their shyness caused most of them to leave strategic positions and gather in a corner of an underground grocery store, where they eventually got ambushed and destroyed in a moment. But while on the bridge, one Taman disappeared from the sight of my soldiers. After a time, I had sent one my Agent into the water, and I found the Taman, how he truly ingenously and bravely tries to sneak on the other side and attack my guys from the back. I really can evaluate such a valor, so I ordered the Agent to salute him, and send him, on Phalanx's expenses, on a cultural excursion to a local place called Heaven.
So, what do you think a realistic conflict with aliens would look like? This game is really great, and it truly can lift up an earthlings' self-confidence, but I'm afraid it isn't a probable scenario of a conflict with aliens. It is rather a tale, about an ingeniosity of human spirit, of a struggle against an attack, of an ability to learn, survive, and win, of turning enemy's weapon against themselves. But one thing I know for sure - we would surely win in storytelling!
