Activate Bristly Bill by paying ({3} magic symbol) ({G} magic symbol) ({G} magic symbol) , doubling the number of +1/+1 counters on each creature you control.
Activate Devoted Druid's first ability by tapping it, adding ({G} magic symbol) .
Activate Devoted Druid's second ability by putting a -1/-1 counter on it, untapping it.
Repeat from step 2 four additional times.
Repeat from step 1.
If Devoted Druid has six or more +1/+1 counters when you begin the combo, it will also get infinite +1/+1 counters which can be removed for infinite green mana.