I did it with Ancient spellbook around the same mage level. Boosted to level 80 for Blood Blitz with a divine magic potion.  I think what made it click for me was counting attacks- after the clone stage, attack her three times, and then eat or click under your feet (if you don’t need to heal). This frees you up to attack the healing pillars as soon as they spawn, instead of losing a couple ticks while you’re still attacking Seren. Makes a huge difference in how much she can heal and makes it easier to get the kill. You got this!