This can be spread across any number of games. Its easy to lose pack members when battling an epic, as they will foolishy fight toe-to-toe with the creature.
Creatures summoned by omnivores also count as posse members.
Advance far enough in the stage to where you are able to have a posse of 3 pack members. Recruit 3 pack members, take them near an Epic, attack and sacrifice them to the Epic, and repeat.
This achievement can be earned in Spore Galactic Adventures by creating an adventure, making the adventure set on a lava planet, giving your captain a crew member and a starting point then testing the adventure. Lead your crew member into the lava and they will burn to death. If you do this 30 times then you will get the achievement.
If the player is already in the Galactic Adventures Stages with his or her creature, this is still achievable by equipping a Shaman captain with the Hypno-Melder which will give it the ability to make temporary posse members out of enemies. After hypnotizing an enemy, the player can then lead it to its subsequent death by its own teammates or the environment, if the AI is unintelligent enough to stand in lava so long as the player is nearby. The player can also set a creature's health to one, hypnotize it, and then throw a handheld object that does damage when thrown, such as an axe, at it. The creature should respawn to make the process quicker. Killing posse members while Test-driving also counts.
Use the Adventure editor, and add 3 crew members. Add mines around the player, and make them 2,000 damage, then lower radius so they don't kill the player. Then, test it. Your allies are killed instantly. Go back to editor, and repeat. This is a VERY fast way to get this achievement!