Best storm mage is probably Acee. If you want both storm and water, I would use Acee, Snipper, and either Umberwood or Wildwood. The two woods have the same physical attack and defense at max level, and Wildwood has a slightly lower magic defense but higher magic offense. The wildwood also has more offensive tricks, including Earsplitter, but you'll mostly be using them as tanks when you're defending anyway, so I personally picked Umberwood for the higher magic defense. ---
With this team you have 2 familiars that can cast Upsy Daisy both controlled by Swaine who never uses MP and who has the highest defense (last of your party to fall in battle most likely). You can always keep Oliver/Marcassin going with this setup while saving Oliver/Marcassin's MP for healing and offensive spells.
This lineup also allows you the flexibility to launch basically any kind of magic attack with any member of your party in case someone gets knocked out/asleep/otherwise unable to act.