Players should remain mindful of cover in XCOM 2. The AI instantly targets squad members that are out in the open -- and they’re much easier to hit -- so you’re just asking for it if you don't pay attention to your surroundings. Cover is represented by either half or full shield icons, providing partial or full cover, respectively. Obviously, full cover is ideal, but oftentimes partial cover is more than enough if it means you can get an angle on your enemy. These icons might change, or disappear, as enemies move around the map.
Once you are in a position to attack, try to flank your enemies for an increased chance to hit -- possibly with a crit for bonus damage. Some soldiers also get a bonus for attacking from above, so try to get to high ground and behind cover whenever possible.