Raiden X is an excellent Flash game that is a lot of fun to play, though it was a bit easy for me to complete on Normal difficulty. The trick is in the power-ups, which are many. Small squares that change in color from Red, to Blue, to Purple will float around the screen after blasting certain enemies. Each color represents a different weapon. My favorite was the Purple shocker that emits a powerful arc of electricity that seeks out enemies on-screen and zaps them until they are fried. Wait until the power-up is the color of your choice to upgrade its power, otherwise your weapon will change.