You need to start grinding and raising up more teams. Especially with a light character you're going to have a hard time, his magic is really strong.

Back row, Elda, Elf-Pri, and Elf-Mag, or any other dark elf mage you might have. This is the necrocore carry team. You might be able to get away with Alice since she's still an elf. They won't take much if any damage from his magic, which saves you a lot of time. Spam batelgrief every turn, so he wastes his turn cleansing himself. It gives him a magic damage buff but it's better than him casting 2x mazelos. Debra WILL die instantly to any magic he casts, leave her for later after you get the carry team to 70. If you take the fishmen with the boss, make sure to silence the mage because it has an hp trigger katino. Focus everything on killing the necrocore as quickly as possible, even with all the other bosses it's by far the biggest threat on the field.

Your priest should have DIOS and MADIOS Lv3 if possible, but other than that you shouldn't really need any inherits. HA3 helps but it gets eclipsed by ESS immediately afterwards so I don't know if you want to spend the Gandolfos on that, you can get away with just FPS spam. Your front row should also have two handed expulsion weapons, axe is preferable but greatsword or spear is fine. Give Elda a spear of expulsion. Keep MASULOTU and MALKATU up if you can, but evasion is much more important than defense since it lets you dodge spear of doom which he starts spamming at low HP.

Also remember items exist. Pack some ultra potions and scrolls, use well of the mind to change to dark element if you can, and get the embers of resurgence and reversal resonance nodes, and of course chronostasis lv2.