Assemble your player and then shoot the enemies before they shoot you.
To build your player tap on the correct body slice and build him up in sequence.
Once your player is assembled, the next step is to shoot the enemies before they get to you.
Press the left side of the screen to shoot the enemies on the left and press the right side of the screen to shoot the enemies on the right.
To protect yourself when the enemies shoot back simply take your finger off the screen and your player will duck down automatically.
Good luck! (you need it)