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, Tribute to the World Tree triggers, putting two +1/+1 counters on the Elemental.
Activate Ashnod's Altar by sacrificing the Elemental, adding ({C} magic symbol) ({C} magic symbol) .
When the Elemental dies, Resourceful Defense triggers, putting two +1/+1 counters on Marath.
Repeat.
Once you have infinite mana and +1/+1 counters on Marath, you may use its ability to deal infinite damage and/or get creature tokens or +1/+1 counters.