This level is very quick and you can bypass most enemies by transforming into a human.
Head to area B from the area A2 (to the right of the transerver, a door on the background) and shoot everything in your way. If you want, at the very end of area A2 there is another door that heads to area C1. Get to the main part of C1 because the room you enter is just a room inside area C1 and head left. You will find a transerver that the left leads to area B2. If you head right from where you are, you will reach the boss.
This boss isn't too hard. His only attack is dropping a stack of three blocks, which will shoot bullets at you. In order damage him, wait until he is right over the blocks he's laid down, then shoot two charged shots at whichever block is red when the orb in the middle opens, making the other blocks fly up and hit him. Do that four times and it will be destroyed.