You can get a new weapon for a short amount of time by getting a PowerUp orb. Each orb has a different color and each gives you a different gun. Once you have a new gun you only have a certain amount of time to use it. The most powerful guns have the shortest amount of time. To kill a Boss, you shoot him until he dies. You can also get more points/life by getting a Life orb. Life orbs have a little white dot inside them and come in 3 different colors.