When you receive Tess, train her until you get the spell Feeble I or II, and Feebos. Once you have received these spells, they will be very helpful against tough enemies and Bosses. When fighting a Boss that does a lot of damage and you want to weather it, use Tess or Deliza's spell to weaken their attack greatly. For a single Boss, use Feeble II. For tough enemies, use Feebos.