You will fight Marcus in a cathedral, on the bottom floor first. On the left and right sides of the room are switches, one on each side. These switches activate bells which disrupt Marcus' concentration and leave him vulnerable for a short time. Hit one switch, then run to the other switch, hitting Marcus on the way. Continue this until his "puppets" disable the bells. Go up to the second floor. Marcus will just run around until you kill him. Use the jump ability to alternate between sides, hitting him on the way. Once he dies, you obtain the Charm ability.

Just hit the bell switches and hit him when he is stunned. When his puppets disable the bell, you can still ring it by jumping into it. He is easier to locate that way. Then, just run around killing enemies while hacking up Marcus, occasionally ringing the bell. You can avoid the puppets altogether, but you will want to build up your Lore bar as much as possible.