To defeat Kraken, use zidane, vivi, steiner and dagger. First, have vivi use thundaga on all its body parts.
This causes it to use waterga on itself,
and sometimes uses ink or water gun afterwards.
kill the tentacles, this stops it from using ink and water gun, and its only two attacks now is a tentacle attack and waterga, it uses waterga most often, sometimes to heal itself.
DO NOT EQUIP VIVI'S RETURN-MAGIC ABILITY, IT CAUSES A REALLY STRESSFUL BATTLE!!!
Use thunder attacks and this battle should be easy.