Oh, you didn't played it before?
There are plenty of ways to play, so no good strategy exist. Personally I stick with 1-2 people, relatively high level (so I could pay for week or some), like Sidney and Raven. Third is the generated guy. Then, after few first cities, I go for top tier guys, but slowly, when money allows.
I don't think that medic, tech or explosive expert are required from start. Explosive guy isn't really required at all, you can easily do almost everything in game without him, although having one isn't bad (and sometimes is fun). For medic, "free" people from resistance (first girl from second sector or something like this) are ok at start. They don't even need to move with your squad, maybe only in first third of game. Tech is same, but it matters less - there always many weapons, and they are changing faster than breaking. In first part of game weapons are pretty limited (until you starting to get rifles), but tech doesn't help. Doors and chest mostly can be opened with brute force, except some specific places.
And in late game you already would get any mercenary you want, so it doesn't matter. But it is only my thoughts, big team with low-level guys is ok.
Personally, I recommend getting the fast transport asap. Don't want to spoil much, but it seriously helps. And training militia to protect cities is a good thing. Order of cities... I always go sweeping from right to left, slowly going down.>On the field I've got tactical knowledge from other games and books, but I dunno what is possible to do. I see that during enemy movements my guys can interrupt and fire shots at them, that's good. But I dunno how cover works or anything.
If you played something like original xcom or such, you'll be ok. Cover matters, but not much as in some other games (sitting behind tree helps of course). Laying or crouching definitely helps. Visibility matters, especially at night, night combat sometimes looks like another game. It isn't stealth tactics, but stealthy way is very fun. If you encounter some unbreakable place (like SAM base) at start, try it at night, it may be easier (but sometimes it is harder, especially when you'll get long range rifles).
Some trick, not overpowered, but often working: if you got some real problems in sector, try to go on roof and lay there, covering all edges. Enemies will spend some AP to climb, and will be easy target when they are on roof edge, especially at night. Shooting from roof is also fun. But, of course, elite enemies may climb and shot everyone, if they are lucky and you aren't.
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