During the battle, one of the key parameters in dealing damage to your enemies is armor penetration. The higher this parameter, the more your damage ignores their armor. To pick up or to restore armor, step on the bulletproof vest icon on the battlefield. The amount of maximum armor can be increased by leveling up your heroes and their abilities.
A first-aid kit can restore some of your health. To restore your health, tap on the first-aid kit icon on the right side of the screen. The higher the level of the hero you have, the more first-aid kits you can use in a row. Your heroes are vulnerable during healing — they cannot defend themselves, and their movement is slowed down. To interrupt the healing process, tap on the first-aid kit icon again. It's also important to remember that your health is restored gradually, not instantly!
Note that your heroes such as Arnie, Cyclops, Sparkle, Hurricane, Levi, Doc, Satoshi, Stalker, Shenji and Vi have the ability to restore armor and health not only to themselves but also to allies if they are within the radius of the ability.