This deck guarantees 8 rewards every time, unless you serious screw up. Summon the Labyrinth turn one and use the Rain of Mercy and Golden Ladybug cards to get at least 9500 LP. Use Cost Down to fill your board with 3 Labyrinth Walls in defense. Use 1 Pot of Benevolence to refill Mai's deck so she doesn't deck out. Use the remaining Pots to either stall to get your LP high enough or return an accidentally discarded piece of the last-turn combo into the deck. On the last turn, play Double Summon, Winged Dragon of Ra, Gravekeeper's Vassal, Secret Pass to the Treasure, and Union Attack.
You can replace Pot of Benevolence with card draw spells, but you risk losing them if Mai gets an Amazoness Sage out on the field. You can use other LP generation cards, but with this setup you only need Golden Ladybug to proc 5 times before you can use the 3 Rain of Mercy cards to reach 9500 LP.
+1000 (100 LP left from Ra's effect)
+500 (using Ra)
+200 (Ra is prismatic)
+3000 (10k damage)
+400 (spells and tributes
+1000 (using a god card)
1 The Winged Dragon of Ra
3 Skelengel [Ultimate Rising R]
2 Golden Ladybug (don't have 3)
3 Rai-Mei [Neo-Impact N]
2 Labyrinth Wall
3 Dian Keto the Cure Master [Tea SR reward]
1 Shooting Star Bow - Ceal [Neo-Impact SR]
2 Ancient Rules [level up Mokuba]
3 Solemn Wishes [Tea SR reward]
Pull ladybugs first with Rai-Mei. Drop Wishes after Mai fills board so she probably won't play Sage.