Ok, the combo works like this.With
Devoted Druid you tap it to get the 1 green mana floating. To untap
Devoted Druid, you can put a -1/-1 counter on it. So when you do that, you use your floating green mana on
Quillspike's ability. Which allows you to take the -1/-1 counter off of
Devoted Druid, and
Quillspike gets +3/+3 until end of turn. You continue to do this until
Quillspike is big enough to kill you opponent in 1 attack. But, don't attack. Use
Rite of Consumption and sacrifice
Quillspike to kill your opponent easier.