Stock up on grenades and medkits before starting a mission.
Team
Keep your team together during missions. If you have to split them, only do so if you have enough skill and firepower to keep them up and complete the mission faster. A team made up of a Medic, Breacher (for grenades), Fireteam Leader (for buffs to nearby teammates and Draw Fire), and Weapons Sergeant (for suppressing fire) is a good combination. Use cover whenever possible to protect your team. Make every destination when moving your soldiers to a location that provides cover. If you moved to the wrong square, select and hold on that position then drag around to set a new destination. When leveling your soldiers, consider their role and focus on stats that benefit them the most. Have your team cover as much space as possible. Only enemies that are within their line of sight can be shot.