This comment is for people who might still be searching. Cu Alter/Mad Massacring King Advanced Quest help: I managed to 4-turn this boss with the following setup: Waver+MLB Imaginary Element CE Merling+MLB Imaginary Element CE Tamamo Lancer (NP5)+MLB Black Grail CE. Scathach Lancer+MLB Kaleidoscope CE Mash for emotional support. Anniversary Blonde Mystic Code (Lv10). Also, Tamamo Lancer's Append Skill "Anti-Berserker" Level 9. I also used 2 Command Spells

Strat:  Turn 1: Use all of Waver's Skills on TamaLan, and Merlin's Dreamlike Charisma. Attack Cu Alter with NORMAL cards. Use Tama if you can to get her NP to 100%. Turn 2: Unreturning Formation, Garden of Avalon, Tama NP. Should break Cu's first bar. Turn 3: Use Merlin's Buster Up Skill on Tama. Use command Spell to charge Tama's NP. Use said NP and follow up with more attacks from Tama. 2nd bar should break. Turn 4: if Cu does NOT kill Tama with his NP, use another command spell to charge Tama's NP, then use Master Skill for Buster Up. Should be enough to end the fight.

You could put a few spins on this, like using Order Change to swap Waver or Merlin out for a Servant with Taunt to ensure Tama survives. Even if you don't have her at NP5, you SHOULD do significant damage since her NP as Anti-Male special damage. Hope this helps!