Strings uses a Slifer Deck, which focuses on getting cards in Strings' hand to increase the ATK of "Slifer the Sky Dragon" to terrifyingly high levels, making it virtually invincible. Other than that, it appears to be a Slime Deck, focused on defending until Strings can draw and Summon "Slifer". The amorphous forms of the Slimes could represent Strings' mind being formed and shaped by Marik.
Strings' ultimate strategy is to get five specific cards out onto the field, which, in the manga and original Japanese anime, is called the "God Five" combo. These five cards are as follows:
"Slifer": The main attack force with nearly unlimited ATK, also able to severely weaken if not outright destroy the opponent's monsters even on the opponent's turn.
"Revival Jam": The ultimate defense, to block any attack or counterattack and immediately revive itself afterward.
"Jam Defender": To allow "Revival Jam" to intercept any attack or counterattack aimed at "Slifer".
"Card of Safe Return": Each time "Revival Jam" revives, Strings draws three cards, allowing "Slifer" to gain 3000 extra ATK.
"Infinite Cards": Removes the six-card hand limit, allowing the ATK of "Slifer" to climb virtually without limit.
However, despite Marik's claim that this combo will allow "Slifer" to be completely indestructible, Yami Yugi proved otherwise, as he took control of "Revival Jam", forcing the effect of "Slifer" to constantly destroy the reviving monster, while Strings' own "Card of Safe Return" forced him to keep drawing until a Deck out occurred, thus causing him to lose. According to Kaiba, the combo's greatest weakness is that the ATK of "Slifer" is limited by the number of cards within the player's Deck.
Yami Marik re-uses various cards used by Strings during the Battle City finals.