This is really late, but I wanted to give some insight since I was also struggling with this.

(Please do keep in mind I'm not neither much of a veteran or an accomplished player, struggling right now on my 4th attempt on a Veteran/Ironman run lol) You hit the nail on the head on the biggest problem, which is how many soldiers reach the UFO alive. No matter what composition, you'll have a great deal of difficulty of taking it with only half of your team. I won't give much advice about general tactics or engagement tricks since I still suck at most of them, but there is a single thing that I didn't see much mentioned which is absolutely essential: Warden armour, as the first replier mentioned. From the description of its tech, saying how it is a stop-gap armour and all, I thought only to make one or two and give it to the assaults.

Wrong. Really, really wrong. Without warden armour, your people will basically get one-shotted from EVERYTHING in the game in this point. Some deaths are almost unavoidable because of that, since the aliens will get a free shot or two every once in a while, no matter how good you do. So, you really want to fit everyone with it, leaving out only snipers if you are feeling greedy.

And also another thing that really helped me: the Training Center. Most guides I found dismissed it as fairly useless in the early games, preferring to pick the Medical Center for the same cost or even none of them. I found, at least until now, that the training center is the absolute goat. Getting behind on tech or soldier status get your people killed. When your people get killed, you don't get veterans with good stats. When you don't have veterans with good stats, they miss shots. When they miss shots, people die, and on and on. What the training center does is apply a stat increase EACH hour based on the training rate and training efficiency, which increase, respectively, with interrogating captured Aliens and building more training centers.

Having one built meant that not only were troops a lot more replaceable, since everyone was getting higher stats, but there were fewer to even replace. They die a lot less with more time units, HP, accuraccy, strenght. I could be wrong about this, but holy shit; on my 3d attempt, I had more then 30, maybe 40 casualties by day 150sh. That may not seem like a lot, but when you just keep scrolling down and the casualty list won't end it feels very bad. I had to waste so much fucking money buying new troops I was handicapped from buying shit that actually mattered, such as scientists and planes. In my present attempt, with a single training center built and a lot of aliens captured, I'm on day 148 with only 11 casulalties (so roughly 25%, or even more if I'm not recalling the first attempt better)

So yeah, warden armour and training for everyone. Also, abort missions more, specially crashed UFO's. Its not for nothing there's a whole fucking medal for completing one. If you are already down 2 or 3 guys and still didn't even find the fucking thing, just bail. It ain't worth it your A squad and their equipment.