Different fighting scenarios

Here are some scenarios of position of my team. Could you please tell me what to do:

Say I am a mage/assasin who is neither fed nor behind. It's a full 5v5 fight. I can't get to their backline because amumu and nasus are in front. What do i do? Should i just use my burst on the front line? should i wait until i have a way to go to backline?

Say i am a bruiser. 5v5. Say i am wukong or irelia. Should i use my gap closer and go for their most fed mid/adc or should i help my adc/mid and peel for them with my cc and dmg?

What should a bruiser do? what is their job in a full 5v5 fight? how about in a fight where we are at a number disadvantage? Same for mages what should i do, what is my job as a mage/assassin?

How do I win a fight if I am a fed (annie/orianna) but I simply cannot get to their backline and their front line hurt my team very hard as bad as the backline hurts?