Activate Marath by paying ({1} magic symbol) and removing a +1/+1 counter from it, choosing to create a creature token.
Unbound Flourishing's last ability triggers, copying Marath's ability.
Resolve both Marath abilities, creating two 1/1 Elemental tokens.
Activate Ashnod's Altar by sacrificing an Elemental token, adding ({C} magic symbol) ({C} magic symbol) .
Activate Marath by paying ({1} magic symbol) and removing a +1/+1 counter from it, choosing to place counters.
Unbound Flourishing's last ability triggers, copying Marath's ability.
Resolve both Marath abilities, putting two +1/+1 counters on Marath.
Repeat any number of times.
Once you have infinite creature tokens, you may activate Ashnod's Altar any number of times for infinite mana.
Once you have infinite mana, you may activate Marath any number of times, placing any number of +1/+1 counters on it due to Unbound Flourishing, and then on any number of other creatures as well.
Once there are infinite +1/+1 counters on Marath, you may activate it any number of times to deal infinite damage to any targets.