Instructions
Put Slifer the Sky Dragon and Obelisk the Tormentor into your deck. You obtain Slifer the Sky Dragon after defeating Yami Yugi and Obelisk the Tormentor after defeating Seto Kaiba.
Put light, dark, earth and wind-type monsters into your deck. Marik uses dark, fiend, earth and wind-type monsters in his deck, so you will want to load up your deck with the aforementioned types of monsters to counter his monsters.
Use trap and spell cards one at a time. Marik has three Harpie's Feather Duster cards in his deck which can instantly destroy all trap and spell cards on the field of play.
Attack Marik with dark-type monsters. Throughout the battle, the field of play will be under Marik's Yami Field, which gives a boost to dark-type monsters. This works for Marik, but it also works for you as well.
Summon Obelisk the Tormentor or Slifer the Sky Dragon. Both of these Egyptian God cards are strong enough to defeat Marik's monsters. Both of these cards require you to tribute three monster cards in order to summon them. When summoned, Obelisk the Tormentor has 4,000 attack and defense and negates all spell and trap cards on the field. When you summon Slifer the Sky Dragon, his attack and defense are equal to all of the cards in your hand times 1,000.