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
Reiterate to your hand.
Repeat from step 3.
Notes
The cost can be reduced if
Molten-Core Maestro has higher power.
Results
Infinite +1/+1 counters on a creature.
Infinite copies of the casted instant or sorcery.
Infinite magecraft triggers.
Infinite red mana.
Infinite storm count.
Metadata
In 113 decks according to EDHREC.
Bracket Tag
This combo has been estimated to be Spicy (Bracket 3-4+).