Actually not just easy, this one was super easy lol.
Plant + Lightning = Cactus Dragon
Yes, literally that simple, again another one, that I don't think you can go wrong on :) And as you can see, one of the many new goals in Dragonvale.
If you want to add me in the game my gamecenter name is prplrush. Feel free to follow my blog for more updates, via google friend connect. (right below this post) and google + this post