First of all never pick up the missiles weapon because you can't control where the missile will hit and you don't want to hit the bosses until you clear the stage because you will end up with a lot of projectiles and tentacles.

For the first 4 bosses respawn them on the right to have some space on the left to kill first the shield blobs and then the bosses. On every boss the shield blobs multiplying x2.

Now, begins the hard part! From the 5th boss there will be also start appearing the buffaloes multiplying x2 on every boss along with shield blobs. So, now you have to respawn the bosses on top left/center off the screen. First start killing the first pair of shield blobs then the buffaloes and repeat this process until you clear the stage and then kill the bosses. If you will not follow this method then you will end up with a stage full of shield blobs or buffaloes and eventually you will lose.

When you kill the 10th boss leave 1 tentacle and wait when the laser weapon will appear then hit the tentacle and pick up the laser to instantly kill the boss and now you have to survive until the laser or the slow down weapon will appear to kill the 12th boss.

GOOD LUCK!!!