If the party has Summoner's Abate Lightning, they can reduce DeRosa's Thundara damage as he often casts it. Edea will start the battle with Charmed status, so the weakest party member should attack her or use Esuna to heal her. The player should be mindful when chaining many attacks together as one can quickly max out DeRosa's BP, leaving one vulnerable to a string of physical and lightning-based attacks.
If the party obtained the Light Curtain from the Land Turtle before, they should use it on DeRosa so that when he casts Cura on himself, a party member will be healed instead. While the Light Curtain won't last many turns, DeRosa will nevertheless not be able to heal as much HP as he normally could have, making him easier to defeat.
The party should use all items and skills available that reduce damage to lightning. The Spell Fencer's Magic Armor support ability reduces damage on characters when Thunder or Thundara are cast on their weapons using Sword Magic. Additionally, the Spell Fencer's Anti-Magic speciality will further reduce magic attacks after the Spell Fencer has been hit by a spell. The Thunder Charm accessory is also another good option to reduce damage from DeRosa's Thundara attacks.
It is also helpful to have a character other than the healer be a Time Mage or have Time Magic as a sub-command. Since DeRosa's BP can add up quickly, casting Haste on the healer will expedite their turns, allowing players to heal to full health before DeRosa attacks. It is not recommended having the healer also be the one casting Haste as this eats up an additional turn which could be used to heal characters or to cast support abilities such as Protect and Shell on characters to further reduce damage.