Based on what you've told me I tried my best.
Just throw in whatever big fusion monster + Riryoku I guess instead of Master of Oz. The thing with Oz is that don't need an extra card (Riryoku in this case) to hit the 10k damage mark, since 800 ATK from the Piranha + 4200 ATK from Oz gives Piranha 5000 attack with Martyr, for a 10k damage with Piranha's effect.
So instead of Oz: summon big fusion monster turn before or on the last turn -> summon Piranha on the last turn -> use Secret Pass on Piranha -> use Riryoku on fusion monster -> use Martyr on fusion monster and u will be set for 10k damage.
Appreciate it! One final question:
What is the best way to get drop skills? I'm missing harpies hunting grounds for Valentine, and apparently destiny draw for Yugi. Is it really just finding a deck to farm LDs, and grinding until it magically drops?