With Coat of Arms and Devoted Druid on the field:
Tap Devoted Druid to add a G to your mana pool
Cast an elf, increasing the power and toughness of devoted Druid because of Coat of Arms
Untap Devoted Druid and put a -1/-1 counter on it
Repeat
With Coat of Arms and Devoted Druid on the field:
Tap Devoted Druid to add a G to your mana pool
Cast an elf, increasing the power and toughness of devoted Druid because of Coat of Arms
Untap Devoted Druid and put a -1/-1 counter on it
Repeat