The meaning of "Түмний баатар" differs in which context you use it. It can mean 'national hero' or 'a knight who leads a thousand soldiers'. "хашраах" means 'to punish' or 'to teach someone a lesson' and "хэрэгтэй" is 'should', 'have to', 'it is necessary', 'it is worth' etc.. I think in your context it probably means '(we) should punish (someone)'