Killing one enemy repeatedly is not difficult, given you come prepared.
When I say 'prepared', I mean your tank has to do his best to keep hate off the Black Mage, Samurai, and Ranger (difficulty); Your Thief has to be quick with SATA and use status bolts (depth); Your mages have to monitor their output closely so as not to pull hate (difficulty); Your Red Mage should employ every applicable debuff to the mob while also serving as a backup healer and nuker (depth); The experienced Bard should be able to keep the action going without interruption (gameplay).
If you do not have a majority of these qualities in your group, then killing crabs over and over is going to be a long and painful process. Of course, killing the same mob repeatedly is what we do in this game now, so I'm not sure what point you're digging at.