The Giant Crab is a boss in Final Fantasy Crystal Chronicles: Ring of Fates, fought in the River Belle Path. It is based on the Giant Crab from Final Fantasy Crystal Chronicles.
A team is quite helpful as the battle will likely be difficult regardless of one's level.
Also, it's a good idea to cast Barrier (combining Clear and Fire) on oneself and freeze the crab frequently so it can be attacked repeatedly.
Note that it is possible to beat it before going to Mount Vaal in Story Mode.