Cryo-chamber
When your mercs are killed, they will be sent over to the cryo-chamber, which is an offshoot of the "Squad" section. They can be revived, which will be free, but it will take a few hours to do. Until then you can complete missions with your newer (weaker) mercenaries until they revive, or just close the game until they are finally done. But they'll keep all of their rank-ups when they are revived.
Gameplay Strategy
When you're in battle, you have to find a good balance between getting in an offensive position where you can maximize the accuracy of your shot, and a defensive position where you can avoid damage. If your character's body has a blue striped line over it, that means they are protected. Make sure to check the accuracy before you shoot at an enemy so that you know whether you're likely to actually make the shot or not. Anything under 60 percent, and you might want to change positions.
Move slowly and keep your squad together as much as possible. They should be balanced with a scout, a heavy and a sniper. Move all of them a few spaces at a time, so that you can encounter enemies more slowly rather than all at once. It'll take more turns, but you have an unlimited number of turns in battle, so that doesn't really matter.
Magnifying glass button
If you want to unlock a piece of equipment in the armory or in the science bay, you have to have the right parts for it. If you don't have the parts, tap on the equipment and hit the magnifying glass button. The world map will pop up, with a "!" right over the area that you need to go to in order to earn it. Some stuff drops randomly, while other stuff will drop when you complete a certain percentage of that area.
Upgrade mercs
You can upgrade your mercs' merc skills starting at level 6. You need skill points to do this, but various statistical attributes can be improved this way, new special attacks can be added, and other buffs, such as avoiding specific types of status-draining attacks, can be activated from this menu.