Just as the achievement says, kill the bodyguard Captains of a Warchief then call him out and kill him before any other Captains step up. Do this twice and the achievement should pop.
Your best bet is to kill the bodyguards of a single Warchief then kill the Warchief, if you leave too long between killing the bodyguards and killing the Warchief, other captains will step up.
If you initiate the Warchief fight and and he spawns with a Captain, that fight won't count towards the achievement.