As with previous incarnations of the game, winning isn’t easy. Here are some of the strategies and tactics I’ve utilised so far. With seeming success. I should clarify this is for the vanilla XCOM2 without extensions such as war of the chosen.
Strategy
Dont delay making contact with other regions. You will need the access to destroy alien installations and delay the Avatar project.
Don’t build relays on every region but position them to create access to other regions.
Don’t panic as the Avatar project progresses - just keep pace. Completing the storyline will win regardless of where the meter has got to.
Prioritise gaining engineers and shuffle them between Avenger tasks when needed.
The “UFO hunts Avenger” event I found really hard, avoid where possible especially early on without defensive turrets (unlocked through researching turrent wrecks). Using a sneaky ranger to skulk round the outside of the map to get into a shooting position on the alien tower then attacking along with snipers+squadsight. Sorry that was more tactical.
I kept a small roster of troops, two of each class, including psi. Aimed to get one in each class promoted fast. Especially in the early game frequent injuries meant some strange squad compositions but it is manageable.
Tactics
Use the roof.
Concealment isn’t designed to allow you to sneak across the map. You might be able to but there seems to be a mechanic where the patrols start to converge on you anyway. You will end up with a nightmare of multiple enemies all at once. Use it to sneak forward and identify an initial patrol to ambush.
Many missions will trigger a final action as you complete it such as troops dropped near your evac zone when you plant the X4 explosives. Don’t execute the objective until everyone is good to go or covering that incoming squad.
Two skilled snipers can be insane.
Use area attacks to reduce the hit points of enemies en-masse then polish all of them off with a sniper using the serial ability or a ranger with the reaper ability. I’ve had a ranger slice down 3 mutons in one turn.
I tend to prioritise upgrading weapons over armour. When you hit the aliens you need to drop them quick, hence weapons. Avoid getting shot in the first place.
I used the hacking specialists a lot to gain control of alien MECS, turrents, sectopods etal. It effectively gives you an extra disposable squad member who ‘appears’ in the mist of the enemy. Also useful for scouting ahead and deliberately triggering aliens and getting them to come to you.
Ditto using psi operatives and stasis on an enemy to allow you to concentrate fire on the remaining targets. The operatives domination ability can be hilarious. Once gained control of a gatekeeper - that mission didn’t last long.
The mimic beacon is a life-saver. Not just as a distraction but as a lure.
Once used a single ranger to move across the map, plant the X4 and escape. What a hero.
Overall it’s about controlling the fight, crowd control and using weapons and abilities at their most effective. So for example, use an EMP bomb to damage or shutdown a sectopod. Hack it and shutdown if needed. Kill the accompanying troops (quickly!) and then whale on the dozing sectopod with things like a grenadier with chain ability and snipers with bluescreen rounds.
The final mission is as you might expect tough. Take it easy and preserve your weaponry as much as possible. On reaching the final chamber I triggered the reaction then ran like the clappers back across the bridge. By using effects like mind-control, rift, hacking and grenades I was able to create an effective kill zone. Only the avatars teleported past there to close with the squad which was convenient: kill them as a priority.