The 'Stinking Rebels' achievement is earned when a warchief with 5 branded bodyguards is betrayed in combat.
Warchiefs may have up to 5 bodyguards, if they don't you can brand a captain and command them to perform an initiation to a warchief which if successful will turn them into a bodyguard. In order to command a captain to perform an initiation, you must grab (RB), dominate (A) and command (Y). In the command menu select the target warchief and the option for initiation should appear. The captain will then have to successfully perform the initiation which is a random mission such as hunting or combat trial.
Once a warchief have 5 bodyguards, ensure all 5 bodyguards are branded and engage in combat with the warchief. In combat you must then activate the branded bodyguards (up), they should all renege and you'll get the 'Stinking Rebels' achievement. The warchief does not need to be killed to get the achievement, the achievement is earned the moment the bodyguards renege.