Well first you'll need to learn undead mutant in the necromancer class. Second you'll need to be a lvl 10 knight to access the secret mission. Once you have all of these things start the secret misson in the knight class. You will face three demon knights. Just wipe floor with them by using undead mutant. You'll come to the boss after you beat the three demon knights. Just use the same technique and wipe the floor with her. Beware though she has an annoying skill. She can take your potions and use them against you. So only bring a few if you think you'll need them. She also has a very powerful darkness attack that is 2-3 hits. You will get healed after the first battle and before the last. Also you need a good amount of mana to constantly use undead mutant. It takes up 125 mp but it's worth it. Hope this helps! Also Devil Never Cry is not responsible if this techniqu doesn't work for you. What do you want for nothing. But really it works and the rate you Z-tokens is amazing you usually get them after the third demon knight. But don't get your hopes up.