Very simple achievement to get. On your map look for the skeleton icon, then go start and complete one 10 times. Clashes will involve burning tribes to the ground or killing a set number of targets then searching them. Achievement done once you do 10 of them, I did them all at once and took me about 30 minutes with most fast travel paces unlocked.
Easy to do most of these from afar using the Owl and beast to wipe out what you need, even the burn ones can be done by the owl if you have learned all its skills.