XCOM 2 is squad-based games: success depends upon special attention to the military coup d'œil, and the proper use of strategies and tactics. The use of appropriate squad tactics directly dictates mission success or failure.
The Art of the Ambush[]
XCOM 2 introduces a new mechanic called concealment. Most of the time when your troops enter the battlefield, the aliens won't react to them right away due to fog of war. So long as you stay outside of their field of vision, you can maneuver without drawing aggro up to 10 turns on Legend difficulty...
Often the best course of action is to plan a hasty ambush. For best results, stage the rest of your squad with good sight range into the kill zone. Have a few in overwatch, ready to fire when the aliens take cover, attack or flee. Keep a few actions in reserve, and special abilities often do more damage than a standard attack. It is worth mentioning that overwatch fire as part of such an ambush does not carry any aim penalty (contrary to regular overwatch without special perks).
Don't Hack Everything[]
Don't go crazy. Blowing a hack can lose concealment and sometimes even call in reinforcements. Only attempt hacks on robots and turrets once concealment is lost; towers are just too risky.
Know When Fold 'Em[]
A tactical withdrawal is often the best option. Unlike previous iterations, leaving the combat zone is a free action, has never been easier or more rewarding than it is in XCOM 2. Most of the time, once you've secured the mission objective you'll be assigned a new task: Kill all the aliens on the map. But if the objective is portable and your troops are being overwhelmed, then cut your losses!
Keep Moving Forward[]
Nearly every mission in XCOM 2 has a timer, and nearly all of those timers require your troops to be in the right place before the clock winds down. The only solution for being on time and on target is to employ bounding overwatch tactics as you keep moving forward. Only move concealed soldiers up through front-line while the rear line cover them. If those new front-line soldiers encounter an alien while pushing back the fog of war, use rear-line soldiers' actions to pin the enemy down, destroy them or force them to move.