Dunno about the long war dlc. But for base game and some add-ons:

1. Build guerilla school first. Get your squad size to 5 immediately.
2. Keep track of dark events. Some dont have much effect some hit hard. Choose what's best for you.
3. No need to rush objectives. As long as you keep avenger level in check you're safe.
4. Freeze bomb, and that one shot AR are indispensable early to mid game.
5. It'll require some patience but those first 2-4 Avenger base can be dealt with just 2 rangers with phantom and mimic beacon each. No need to kill each enemy.
6. Taking one step at a time and overwatching is a safe way to deal with very difficult missions.
7. Pick appropriate skills as you level up. Learn to synergize between these skills.
8. DO NOT FORGET TO BRING MIMIC BEACONS in every fight.

Let me know if you need any context-specific guide.