So Many Me

As Dabigamc explained perfectly in his solution you need to use the red bouncing MEs to beat the boss with his own ammunition.

I found the last part in which he was running away from you extremely hard. I just could not find a good spot to place my red ME, so that the bullets bounces back and hits the boss.

I found a video which shows a good spot to place your ME. The walkthrough in the video is not the best but at least it shows one consistently working spot to place the ME. I still needed a few tries to get it right, but that was just me messing up the last part of the race. Unfortunately the video is not in English, so sorry for that but it was the only one I found.
He places the ME that kills the robot vacuum at minute 24:35. So feel free to jump directly to this point.
The boss fight itself starts around 15:20. So if you need help on the first 2 parts of the fight you can also jump to this point in the video.
(Video credit: Sazzle)

Hope this helps you guys :-)
Any improvements? Please let me know in the comments below

Story related.

Awarded for beating the Boss on the 8th and final level of the second world Outskirts.

To beat him you have to chase him until you stop and then grab the bouncy ME fruit and use it to bounce his spiky balls back at him. Do this twice then continue and do the same.

The final phase is he will be running away and shooting balls and you must grab the bouncy fruit again and judge where it is going to land and send it back using a bouncy ME. You have to do do this before the end or it will keep restarting you at the last checkpoint after taking away 4/5 health blocks.

Hope this helps