Formal:
* Brawler
* Thug
* Hardened criminal
* Enforcer
* Heavy (as in "He's a heavy in the mob")
Informal:
* Bruiser
* Roughneck
* Goon
* Tough cookie
* Hardcase
* Hothead
* Scrapper
* Strongarm
Figurative:
* Iron man
* Stonewall
* Rugged individual
* Hard-boiled
The best synonym will depend on the context of your writing. Consider:
* Is the tough guy a criminal or just someone who is physically strong and intimidating?
* Is the tough guy a powerful figure or someone who is just trying to protect themselves?
* Do you want to convey a sense of admiration, fear, or both?
Once you have a better idea of what you want to say, you can choose the most appropriate synonym.