Devoted Druid does not have a -1/-1 counter on it.
({1} magic symbol) available.
Steps
Activate Devoted Druid's first ability by tapping it, adding ({G} magic symbol) .
Activate Marath by paying ({1} magic symbol) and removing a +1/+1 counter from it, putting four +1/+1 counters on it.
Activate Marath by paying ({1} magic symbol) and removing a +1/+1 counter from it, putting four +1/+1 counters on Devoted Druid.
Activate Devoted Druid's second ability by putting a -1/-1 counter on it, untapping it.
Activate Devoted Druid's first ability by tapping it, adding ({G} magic symbol) .
Repeat from step 4 one additional time.
Repeat from step 2.
Once you have infinite +1/+1 counters on Devoted Druid and Marath, you may use them to get infinite green mana, Elemental creature tokens, and +1/+1 counters, and/or deal infinite damage to any target(s).