Marath, Will of the Wild | Mana Echoes | Phyrexian Altar

Cards

Marath in the command zone.
Mana Echoes and
Phyrexian Altar 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 from the command zone by paying ({R} magic symbol) ({G} magic symbol) ({W} magic symbol) and commander tax, if applicable, causing it to enter the battlefield with a number of +1/+1 counters on it equal to the amount of mana spent to cast it.
When
Marath enters the battlefield,
Mana Echoes triggers, adding ({C} magic symbol) for each Elemental and Beast on the battlefield.
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 on the battlefield.
Repeat from step 3 until
Marath has one +1/+1 counter on it.
Activate
Marath by paying ({1} magic symbol) and removing a +1/+1 counter from it.
Marath dies as a state-based action due to having zero toughness.
Choose to return
Marath to the command zone from your graveyard.
Resolve the
Marath ability, creating a 1/1 Elemental creature token.
When the Elemental enters the battlefield,
Mana Echoes triggers, adding ({C} magic symbol) for each Elemental on the battlefield.
Activate
Phyrexian Altar by sacrificing three Elementals, adding ({R} magic symbol) ({G} magic symbol) ({W} magic symbol) .
Repeat.
Once you have infinite colorless mana and Elemental tokens, you may cast
Marath, causing it to enter with an arbitrarily large number of +1/+1 counters, and choose
Marath's other modes instead.