Jumbo Cactuar on the battlefield (without summoning sickness).
Easy Prerequisites
An opponent controls a creature.
Mana Needed
({1} magic symbol) ({R} magic symbol) available.
Steps
Declare
Jumbo Cactuar as an attacker, triggering its ability, giving it +9999/+0 until end of turn.
Holding priority, cast
The Last Agni Kai by paying ({1} magic symbol) ({R} magic symbol) , causing
Jumbo Cactuar to fight a creature an opponent controls, and causing you to gain an amount of ({R} magic symbol) equal to the excess damage. You won't lose this mana as steps and phases end this turn.
Results
Near-infinite red mana.
Near-infinitely powerful creature until end of turn.