I just completed this after about 20 reloads.
I think I may have tried this a little too early, it still cost me a casualty and I used 7 medkits completely by the time it was done.
I noticed that if you save before landing it changes the where the aliens spawn at so I did that a few times until I wasn't being ambused by 4 or 5 aliens with RL's right at the get go. After reading this thread and experimenting myself I equipped my squad as follows.
3 - Assualt rifles (Steyr and the HK that fires Hard Rounds)
1- Alien Plasma Gun - uses throwing my soldier with heroic made good use of this
1- Grenade launcher - didn't seem to do much damage but good for clearing terrain
2- Snipers (used the rifle that does 160pts of universal damage)-wore heavy armor
My armor sucked I had 1 suit of Sun Armor, 2 suits of Basic Alien armor, 2 suits of heavy armor, and a couple of Combat Armors.
I tried to use stealth tatics I made sure my squad was set to walk. I set them in a skirmish formation as best I could. I got a little lucky and a couple of Rets were grouped up and I was able to make hash of them with the plasma gun and grenade launcher. One of my faster squaddies spent a lot of time dodging rockets (he gave up walking at that point
One of my snipers was cut down by an Alien with a plasma gun and a few laser shots. At this point the smoke kind of cleared, all of the Aliens on the outside of the wall were dead. I was able to Medkit everybody, reload weapons, and save the game.
My first attempt from this point I arranged my squad in kill position and blew a hole in the wall with a grenade. On the other side off the wall were 5 Rets. They opened up on me with a bunch of rockets and my team died horribly.
My second attempt I arranged my squad in kill position around the gate and then I waited. I was hoping an alien or two would walk through the gate into my kill zone but instead I waited. I set the speed to extra fast and waited about 10 minutes with no one coming through. I then blew through the wall again and the aliens had obviously dispersed, I was able to pick them off one by one. Despite this 2 of my squaddies were too injured and I was out of med-kit so they were left behind.
I took my squad of four in and it was anti-climatic. I took the back way around as suggested in this thread. I killed 3 Rets in transit without much difficulty.
Good Luck... hope this helps some.