Lord DeRosso is a powerful and dangerous adversary. His Battle Thirst attack absorbs BP from its target. Bone Crush deals damage equal to the target's missing HP. Energy Burst deals heavy physical damage to all targets. Graviga deals enough damage to render a target at critical HP from max HP, should it hit.
A rather "cheap", but nevertheless effective strategy involves the Ninja skill Utsusemi. Before going into battle, the player should choose job classes with higher speed (Thieves and Ninjas are prime choices) and equip the Ninja support abilities Transience and Comeback Kid, and the Swordmaster skill Counter Amp. Depending on the characters' speeds, one can fill the remaining slots with either Speed or P.Atk enhancements (as it pertains to this strategy, none of the other stats matter). Equipping Hermes Shoes or Sandals is also advisable, as well as the highest P.Atk weapons available, regardless of whether they are S-class or not. The player can also consider using other equipment that raises these two stats (e.g. Red Caps and Power Sashes).
The goal is to have the party fast enough to perform Utsusemi before DeRosso can attack, and then damage him only through the resulting counterattacks (if one of the characters is not fast enough to act before the Vampire, the player should consider resetting and adjusting either support abilities or accessories to bring their speed up). This will render the party invulnerable to DeRosso's Bone Crush and Drain abilities via evasion, and thanks to Transience and Comeback Kid, one will counter single-target attacks and receive a 25% boost to P.Atk and M.Atk for every attack evaded, respectively. The only attack that will cause damage is Graviga, which he will not use if any characters have low or critical HP (it's also usually the first attack he uses, and it's all but guaranteed to cripple the party into meeting that criterion). Once each character has performed Utsusemi once, the player can activate Auto-Battle and double battle speed and the battle will carry itself out within five to ten minutes.
A drawback is the time it will take for the battle to end. Counters only work on single-target attacks, and DeRosso's favored move, Bone Crush, hits the whole party, meaning that only his infrequent Drain attacks will be countered. If the characters have leveled up the Red Mage job enough however, its BP-granting abilities give the party ample opportunity to do more damage through other means. Though crude, one could counter this effect also by allowing three characters to be KO'd, rendering all of DeRosso's attacks single-target.
Alternatively, one can also use the Time Mage ability Hasten World, which guarantees DeRosso will use his Energy Burst move every turn after the first, and gives an extra BP each turn to use a move other than Utsusemi; and/or use the Red Mage ability Turn The Tables, which will also give extra BP to speed up the fight.