Before the battle trade in your Water Magnus for Fire and Dark. Even though you now have three party members, Gibari won't be much of help/use here, unless he uses Wind Magnus, so let him heal and take pictures. Thunderfish has a four-hit combo that ends with Thundercloud, this attack at this point should be negligible. The Thunderfish can also use Venemous Droplets. While it won't deal any damage, it poisons one character. Apart from this, the Thunderfish really isn't hard. Despite its large amount of HP, you should chop it away with strong Fire and Dark combos. Shadow Wings works wonders in this battle. By now you should have a lot of Flame Swords and Flame Shields, so Kalas should be a good attacker. Xelha can also be deal a lot of damage with Fire Burst Lvl. 2's.