Before That.... How To Defeat dark master !!!???And what Is Partsun rally??? kind of Event?????

You beat the Dark Master by getting slow robots. That's because the chickens' attacks fail when you're facing forward. Also, it reaaallly helps to get a medabot with anti-air parts (ex. bat) since it heavily damages the chickens. As for the frog, you need missile parts (ex. Medabee's head part) since they always hit no matter how many times he uses "conceal".

No, it's not the end. If you've tried to fall in love Erika or Karin (you can only pick one) by doing all their sidequests, you can receive a Cat or a ? medal from them, respectively.

how can I get the cherub arm?

you can best use anti-air weapons against the chickens
because they are fly medabots

ITS NOT OVER YOU HAVE ALOT MORE TO DO THAN THAT!