Do not use any dots at all, its regen is very very potent. And if there is a dot on it once it goes inside shell, it will remain in shell mode for a very very long time thus defeating the effect of what those dots will do.
This fight is all dependent on how long it remains iside its shell. If its in its shell for only 10secs or about 2-4 nukes thats only about 2-4% hp regen if that. But if there is a dot on it, it will stay inside shell for longer nukes, sometimes even the entire mp bar of the blm's and then some and thus regen back to full health again.
Simply use no dots at all or efeebling magic at all.
Bottom line, just melee it into shell and toss magic damages to get it out of shell.