There is an easy way to beat Ashura, the final boss. When you reach Ashura start spinning in circles and blast him when you get the chance. You'll take some hits but if your ship is high you'll blow Ashura away in no time at all.

Select a two-player game and choose your characters. When the game starts let player one's energy drop down to 1 block, then hit Start rapidly on Controller Two while player one loses the last energy block. Player one will reappear at the center of the screen flashing, and he'll be invincible until he grabs energy. This can be repeated for each character.