You don't have to have Merryn fighting the crab from the get-go. Use the sub to bait the crab and, after it opens its claws, quickly circle around to its hindquarters. Then let Merryn out of the sub and have her hit its vulnerable spot with her knife. You can find two of these crabs in an open area with lots of room to maneuver in the northeastern part of Skeleton Reef, fairly close to a save-spot. Kill one of them with the sub, and then go for the achievement with the other.