Marath, Will of the Wild | Food Chain | Mana Echoes

Cards

Marath in the command zone.
Food Chain and
Mana Echoes on the battlefield.
({R} magic symbol) ({G} magic symbol) ({W} magic symbol) plus enough mana to pay for commander tax, if applicable available.

Cast
Marath by paying ({R} magic symbol) ({G} magic symbol) ({W} magic symbol) plus commander tax, if applicable, causing it to enter the battlefield with a +1/+1 counter on it for each mana spent to cast it.
When
Marath enters the battlefield,
Mana Echoes triggers, adding ({C} magic symbol) for each Elemental and/or Beast you control.
Activate
Marath by paying ({1} magic symbol) and removing a +1/+1 counter from it, creating a 1/1 Elemental creature token.
When the Elemental enters the battlefield,
Mana Echoes triggers, adding ({C} magic symbol) for each Elemental you control.
Repeat from step 3 until
Marath has one +1/+1 counter on it.
Activate
Food Chain by exiling
Marath, adding ({R} magic symbol) ({R} magic symbol) ({R} magic symbol) ({R} magic symbol) that can only be spent to cast creature spells.
Choose to return
Marath to the command zone from exile.
Activate
Food Chain twice by exiling two Elemental tokens, adding ({G} magic symbol) ({W} magic symbol) that can only be spent to cast creature spells.
Repeat.
Once you have infinite mana, you may cast
Marath for an arbitrarily large number of mana, and use its ability to put infinite +1/+1 counters on creatures you control and/or deal infinite damage to any target(s).