Thanks for everyone here!
Have been following the forum for about two months. Wish to contribute a little to the great discussion here.
Our game is at the level 24. I just arranged my islands such that all dragons have "+" except
having neither "-" nor "+".
First island: Earth + Fire with boosts (Earth, Fire, Plant)
Second island: Water + Air + Plant (Plant, Reindeer) with boosts (Water, Air)
Third island: Cold + Lightning + 1 Earth (Quake, Mountain, Sandstorm) with boosts (Cold + Lightning)
Hope it will help some here.