Cowboy vs Martians

Big Joe is probably the best cowboy of all history. He has won every gunfight so far and his clean record of victories is something beyond our thinking. Every guy, who challenged him for a gunfight, got totally wrecked. Now, Big Joe will be facing much more dangerous opponents. Aliens! Yeah, you hear right! Aliens came to our planet to challenge Big Joe. His fame must be really something when even aliens are coming to challenging in a battle, but let's get back to the actual issue. Big Joe wil have to shoot down these alien invaders, but his ammo is a bit limited, so he must be very precise with his shots. Can you help him in this uneven battle? Have fun.