This unlocks after "A Dark Ritual", which you earn after defeating "The Queen of Skulls".
You have to Attack the Lich and defeat it to earn its token and unlock the achievement.
You fight the Lich plus a random enemy card. I found it was easiest to defeat the other enemies while deflecting the Lich's fireballs. Once it is alone focus on the Lich, reflecting the fireballs until he teleports next to you, then melee him until he teleports away. Repeat.
It is helpful to have a shield so you can deflect the fireballs and know when to counter.