To command secondary units to use their special attacks, such as grenade throws or heavy gunfire, press the appropriate icon for them at the bottom of the screen, then mark where you want their attack to go.
If you want to use different weapons they should be selected before you go on a mission, and you'll be able to use them in battle.
To use sniper rifle tap the fire button once to use a scope view, line up your target and then tap the fire button to shoot the enemy.
Best way to avoid incoming attacks is to make sure you're out of its firing range.
To level up, complete missions as successfully as you can and take part in PvP battles.
Check out witch supplies are available in the game.