My matchups for the 1st place finish were: 

Round 1 – Yellow Hybrid 2-0 

Round 2 – Royal Knights 2-0 

Round 3 – Royal Knights 2-0 

Round 4 – Miragegaogamon 2-1 

Round 5 – Shinegreymon 2-1 

Round 6 – Shinegreymon 2-1 

Round 7 – Shinegreymon 2-0 

Round 8 – Royal Knights 2-1 

The deck’s strategy is to put your opponent on a clock with sleep mode, forcing them to make decisions they don’t want because Rage Mode will be coming onto the field destroying all their level 5 or lower digimon. 

The primary outlet of cycle with Belphemon is drawing and discarding cards to get powerful effects like BT-13 Astamon’s inheritable to play a level 4 or lower digimon on their turn and floating into your other Gizmons when they get deleted. Psychemon is a strong tech into the Royal Knights matchup as it forces them to remove the floodgate or they cannot extend their plays. Nidhoggmon is also another strong tech card as it acts like a Hammer Spark to extend your plays, or a strong defensive card for when your opponent swings while you have Belphemon: Sleep Mode on field, you can discard 2 cards from your hand to stop an attack, and discarding the Nidhogg will surely make them rethink their next move. 

Belphemon has relatively positive win percentages against majority of the decks of the format except for Shinegreymon. The biggest tip for playing against Shinegreymon is knowing when to Block / Negate an attack since the deck can output massive damage quickly. Stopping the Marcus Sec +1 attacks is usually the key point in survivability. If you can bring out Sleep Mode / Rage mode as quickly onto the field as possible, the safer you are. 

Miragegaogamon feels more like a 60/40, 60 being in Belphemon’s favor, and Blue Flare is the closest 50/50 matchup, as they have tools to stun Rage Mode since it lasts till the end of your opponent’s turn. 

For mulligans, I would only mulligan if you have no turn 1 play. If you have no rookie but you have Eyesmon: Scatter Mode, playing it for 4 isn’t horrible as the On Deletion gets you a lot of value.