Welcome to the Farm Dark Marik Thread.
To farm him you can either use Unhappy Girl and\or Labyrith Builder decks.
Personally I was testing both, and both do the Job.
For instance I'll share My Labyrith Builder Deck:
(Either Blue-Eyes Ultimate Dragon or Master of OZ works for the Extra Deck Spot.)
You get Minimun 7300 Points, WITHOUT Counting Glossy\Pristmatic\ Low LP \ Fusion,Tribute,Ritual Summon.
So to reach 8000 You can Just Add Serket + Temple instead of Summoner and Dimensionhole for example or use Glossy cards Like I did.