Before rushing into any confrontation, you should check a few basic things. This principle applies to the initial assault and each skirmish. Being prepared and knowing what your mercs status is can be important part of any battle.
Supplies
Each area tells you the number of enemies and the level bracket they fall into. Use this knowledge to make sure you have enough supplies to make it through. The most crucial check should be to verify your ammo and medical supplies.
The larger the number and higher the level of enemies, the more bullets and bandages you will need. Also, a large number dictates having some type of grenade ready to deal with stubborn foes.
Lastly, you will want to have equipment to deal with worn weapons, locked doors, and explosive opportunities, such as planting C4 and disarming mines.
Weapons/Armor Durability
The condition of your weapon is shown by the small bar next to it. It is easy to overlook, but if you let it go for too long it might be beyond your ability to fix it. Also, damaged guns, especially when they are in the red, can jam and leave you a sitting duck. The condition of your armor is a lot less obvious, and many times you won’t know how much protection you really have. That great guardian vest you got a few missions back may have taken enough damage to reduce its durability and make it less effective than a flak jacket. Check your armor often and replace it when necessary.
Inventory Management
You inventory has several slots to allow your mercs to haul an impressive amount of gear. As long as they don’t exceed their maximum load weight, they can carry just about anything. You'll find a surprising number of items, ammo, and other goodies to pick up in this game, and there is nothing overly tactical about what you carry.
But there are a few somewhat obvious things to remember to maximize your stored goods.
If a merc is approaching his weight limit, simply switch his heaviest items with another merc who's holding lighter goods.
Make sure to equip every merc with something in their opposite hand (the quick use inventory slot). Even if they can’t use the item, it frees up one more spot.
Trade up on used items. If you have a decent number of med kits and one of them has been used three or four times, swap it out when you find a new one.
Refocus and Reload
The flow of battle usually goes from heated combat to dead calm. It is important to take stock of your situation between encounters. Check your weapons and supplies to make sure all mercs are properly equipped with the ammo and items they need.
These breaks in combat are a great time to reassess your gear and your fighting tactics. For example, swap armor with a teammate if they have low health or if they remain on the front lines. Also, if you are running low on ammo, switch guns before combat because each gun you change out starts with no ammo.
Heal any of your mercs, especially your point man, who is generally exposed to a lot of damage, or if the merc has very worn armor. Make it a habit to manually reload after every encounter.