Phyrexian Altar and Doubling Season on the battlefield.
({3} magic symbol) ({B} magic symbol) ({R} magic symbol) ({G} magic symbol) plus enough mana to pay for commander tax, if applicable available.
Steps
Cast Prossh from the command zone by paying ({3} magic symbol) ({B} magic symbol) ({R} magic symbol) ({G} magic symbol) and commander tax, if applicable.
Prossh triggers, creating a number of 0/1 Kobolds of Kher Keep creature tokens equal to twice the amount of mana spent to cast Prossh.
Activate Phyrexian Altar three times by sacrificing Prossh and two Kobolds, adding ({B} magic symbol) ({R} magic symbol) ({G} magic symbol) .
Choose to return Prossh to the command zone from your graveyard.
Activate Phyrexian Altar by sacrificing a Kobold, adding ({1} magic symbol) .
Repeat step 5 until you have enough mana to cast Prossh.