Smoke
Smoke is VERY powerful at almost every stage. Even if the enemy sniper shoots at you, without LOS it's huge 50% penalty to accuracy. Also, no LOS means to reaction shots.
That being said, it's not that important for saveload enjoyers, as are some things below.
Smoke+dogs/incendiaries is a very effective tactic for taking "Downed UFO" missions at promo I/II. Properly used dogs are OP in general.
Guide-wise. The most important direction of the game, as noted by OP, is getting Promo I/II/III. And along with them, new crafts (van/helicopter/dragonfly/osprey/...), new armors (kevlar/armored vest/...), new labs,
soldier transformations (see available transforms). Check research dependencies for those in tech viewer (q at geoscape) or
wiki.
Another important venue is getting first alien-ish corpse (for alien containment), then live alien, then alien engineer(s)...
Useful for getting through the interface secrets.Soldier transforms are done through base->agents->button below->transformations. The most important one is x-com bio-enhancement: +15 hp, far less likely to get one-shotted by humans. You get it by finding and researching some storm.
Regarding battlescape.
- Most important. Retreat is very viable option throughout the game. Skipping on 40+ chupacabra mission is just a wise thing to do.
- Some enemies have strong armor or resist to a particular damage type. Don't be shy to check the wiki and try to figure out their weakness. The damage formula is, roughly ([a] - ammo/melee weapon, [t] - target):
damage = [a] RNG * [a] power * [t] damage modifier - [t] armor * [a] armor eff
Then there's also corresponding factor to affect a particular target's stat, i.e.:
toHealth = damage * [a] toHealth
toStun = damage * [a] toStun
Using New Battle (from main menu) + debug mode, you can test some weapons/strats directly. - Some enemies have camo, and x-com rat/AI scouts can reduce it's effectiveness. They also see through the smoke fairly well, making it one-sided cover against humans.
Regarding manors. It's not too hard to clear out a manor in general, but it's very hard to do it
quickly. Especially manor with reinforcements, even if you go rocket-blasting and all-out aggressive. And the rewards are somewhat pointless (even though it's points!). It's a chore.
The best way to deal with them is terminating the cult(s), preventing additional manor spawns.
Detecting manors is done is either by getting craft randomly attacked while flying by hidden manor or checking region/country score charts - manors generate some alien score. Patrol with any vehicle around expected manor locations for a couple hours, and chances are, it will pop up.