Same as the other “kill x number of this” achievements, only kill 10 giant crabs. To kill them easily, you should have learned the “kick” skill from a skill trainer; then, when the crabs come at you, simply hold and press to kick them. You’ll flip them for several seconds, during which they are extremely vulnerable.
Alternatively, you could also have Patty in your party (as she has the kick skill), or Venturo (since he can just shoot them with his musket). You could also shoot them with a musket yourself, if you’re following the Inquisition path.