Summon Ojama Knight or Ojama King as early as possible. If summoned early on, he can lock 2 or 3 of Yusei's monster zones making him unable to Synchro summon.
After that just keep stalling until you deck out. Ojama Blue helps you get Ojamas out of your deck, King of the Swamp helps you search Polymerization in your deck.
King of the Swamp can also be used as a substitute fusion material for Ojama King but not for Ojama Knight.
During your final turn, have 2 face-up attack position Ojama Knight/Ojama King then use Ojama Country. Normal Summon Gravekeeper's Vassal then use Secret Pass to the Treasures and 2 Union Attack on him. Direct attack for over 9999 damage assessment.
Remember to use Anti-Magic Arrows during your final battle phase to prevent Yusei from activating his Scrap-Iron Scarecrow.
Defeat level 30 in gate and you should get level 40
Why did they make lv40 Yusei so ridiculously broken... it’s bizarre that I take down his turn 1 Berserker but then next turn he manages to summon Archer AND that other 2000 monster. It’s really ridiculous, what’s the point of playing an opponent that’s invincible unless he hard bricks. There should be an option to block lv40 LDs from appearing in the overworld.
I'm doing well with Spirit Barrier + Astral Barrier farming deck.
I use Neo-Spacian Air Hummingbird + Golden Ladybug to gain LP, Aromage Jasmine + Aroma Garden to draw cards and further gain more LP, and Aromage Rosemary to deal with possible threats by switching them into defense and destroy them in battle.
Then once I'm ready, I tribute my three monsters for Ra.