To take out green dragons, it's better to wait for the Dendroids. They immobilize the dragons, allowing you to shoot them down easily.
I agree with Dirty_Player. I'd add that it's good to have a tactic (preferably an advanced one) to avoid moving with the Dendroids and to maximize the use of the terrain. First, take out one enemy unit with a single attack, and then strike with the Dendroids (otherwise, they will quickly be destroyed).
