Beaten the final boss is 16 or 17 tries
Super cheap boss if you are a ranged unit, you need to go aggressive to have a chance. I tried to run and was always decimated, but when I stick with aggressive gameplay and Ashe of War spamm a finally managed to beat them
For the second phase Golden Braid and Golden Light Fortification kept me alive when the flash lights started popping
Blasphemous Blade did the trick for offensive (use the heaviest armor you can wield to not be poise-break during the fight), very good range and with help of Shard of Alexander great damage output
Beat it in level 228, didn't need to grind or collect additional weapons.