Strongman is a role modifier that signifies that any kills performed by this player cannot be blocked by any means - neither by Bulletproof, nor by Doctor or other protective roles, nor by Roleblocks. It is, however, trumped by roles that prevent the victim from being targeted at all, namely Commuter and Hider.
A Mafia or Serial Killer aligned Strongman is considered Normal on mafiascum.net. A Strongman cannot be stopped from performing a kill, unless their target has commuted. A Bodyguard would essentially be stopped from protecting against a kill and would not die.
Use and Power
Usually, One-Shot Strongman kills are put into the game to balance a potentially broken combination in the game. It is important not to waste the Strongman kill before something like Follow the Cop starts to take place.