Molten-Core Maestro on the battlefield (with power 4 or greater).
Notable Prerequisites
You have another instant or sorcery card in hand with valid targets as applicable.
Mana Needed
({4} magic symbol) ({R} magic symbol) ({R} magic symbol) at most plus enough mana to cast the additional instant or sorcery available.
Steps
Cast the other instant or sorcery by paying its mana cost.
Molten-Core Maestro triggers, putting a +1/+1 counter on it, and adding an amount of ({R} magic symbol) equal to its power if the spell had mana value 5 or greater.
Holding priority, cast
Reiterate with buyback by paying ({4} magic symbol) ({R} magic symbol) ({R} magic symbol) , targeting the instant or sorcery.
Molten-Core Maestro triggers, putting a +1/+1 counter on it and adding an amount of ({R} magic symbol) equal to its power.
Resolve
Reiterate, copying the instant or sorcery and returning