Activate Marath by paying ({1} magic symbol) and removing a +1/+1 counter from it, creating a 1/1 Elemental creture token.
The Elemental enters the battlefield, triggering Mana Echoes, adding ({C} magic symbol) for each Elemental you control.
Activate Marath by paying ({1} magic symbol) and removing a +1/+1 counter from it, putting two +1/+1 counters on it.
Repeat.
Once you have infinite ({C} magic symbol) , you may repeat step 3 to give Marath infinite +1/+1 counters, then use them to deal infinite damage to any target(s).