This achivement is most easily obtained after you gain the ability to brand enemies and to kill all enemies you have branded. You gain the ability to brand enemies a few missions into the second half of the game, when you first brand a warchief, though it may be wise to wait until you gain the ability to use a simple ranged attack to mount caragors, which you can purchase after the graug hunt with the dwarf.

Once you have gained the ability to brand enemies, gaining this achievement is simple:

1) Sneak up on enemies guarding slaves; frequently, if you openly attack them, the slaves will escape while you fight, but if you quietly ambush and brand the slavers, they're much more likely to stay put.

2) Stealth-brand the enemies near the slaves.

3) Keep track of how many slaves the slavers you branded were standing watch over; ideally, you want to only have them stand over 20-25 slaves - some people have reported that extremely large numbers of slaves might cause the achievement not to trigger.

4) Find a caragor and mount it.

5) Ride around and cause branded enemies' heads to explode; this has somewhat limited range, so you may need to do so multiple times.

6) Using your bow, headshot guards who are guarding other slaves to free them quickly without danger to your caragor.