What I have been noticing in my playthroughs is that the aliens tend to 1hit ko your soldiers that are running around long before you spot them,
Most people feel that way, and I sure did for a long time. But with some practice and more than anything understanding the value of reactions, you can greatly alter that. In open and dangerous areas I often use a tank to draw the aliens' fire, but I'll have my rangers run out into some of the safer open areas, such as along walls and fences. Mutual surprise is also very very important. You want to make sure when you're uncovering ground, that your soldier will be able to see any aliens that can see your soldier. You can then use a sniper to shoot them from outside visual range, and they won't be able to reaction fire. Once they're softened up a bit, the ranger can potentially finish them off without fear of reaction fire. When going around corners, I sometimes swing the soldier away from the wall so they can approach the corner diagonally and gain mutual surprise. I'll check the windows before entering the farmhouses, and I'll swing around wide and enter stairwells from an angle that allows the soldier to see up to the second floor. Aliens love to hide at the top of the barn stairwell. In fact it's so common that sometimes I shoot that particular spot from outside.
When going around the most dangerous corners, I use breachers. They're especially useful inside the alien craft, but I can usually explore farmhouses and terror site buildings just fine with rangers.
Another thing to think about is weapons. Snipers may actually do well with pistols, as they can make multiple aimed shots or several snap shots. If you can position your snipers to shoot from beyond visual range, then they can safely injure aliens without taking reaction fire. Snipers can also do well with heavy weapons--one well-placed aimed shot can easily be all you need. It varies a lot by what you're fighting.
I have semi-auto and full auto weapons of every kind in my GWeapons mod. I like to give my rangers semi-auto weapons because those have good snap shots, and I prefer full-auto for the breachers because ideally they'll be close enough that accuracy won't be much of a factor. But I may use the semi-auto rifles because they have a (not so accurate) burst-fire mode, and I haven't done it yet but I'm thinking of using pistols. When entering a medium scout, the soldier may face 3-4 aliens at once. With enough reactions and almost full time units, they may be able to make a shot with a pistol without incurring any or much reaction fire.
Smoke can also play a role. I almost never use it because I'm lazy, but you can use it to get around corners. It can be a tool to enable you to approach the aliens head-on as you exit the smoke, it an environment where you could not have. It's also good for masking your exit from the Skyranger, so your soldiers have time to spread out before they take a blaster. Especially useful with the Bloodthirsty A.I. mod that Meridian made.
Health plays a role even early on. Plasma pistol shots seem to kill unarmored soldiers in one shot most of the time, but a soldier with high starting health actually has a good chance of surviving a plasma pistol shot.
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Announcement: I made a major update to Reaver's Soldier Classes, edit and download are available on the first post.
