How to make ajective from the noun "hero"?
For example:
Jumping into the river to save the girl was such a hero(...?) thing to do.

Question

How to make ajective from the noun "hero"?
For example: 
Jumping into the river to save the girl was such a hero(...?) thing to do.

Answer

Heroic.

Adjective:	
Having the characteristics of a hero or heroine; very brave.

Answer

Heroic has already been answered.

The answer to your second question is : Pointless !

It's pointless to argue with him :)
OR
There's no point in arguing with him