Primordial Ooze is a strong boss. Dash out of the range of his Stinking Missiles, and try to position yourself behind him to avoid the tentacle attack. Using Alicia's Soul Crush is a good idea, as it deals quite a bit of damage. IF you happen to beak anything, dash away, and wait for him to use Regeneration.
After losing 50% of his HP, Primordial Ooze will abuse Lock on and Washout. Have Sylphide heal with Free Item, and go in for the kill. If you got a Flare Gem from the castle, use it now to deal high damage. If not, have one character break off and heal while the others attack.