
blue- base
green- dragons that create the funnel
red- main attack
So basically the idea behind the funnel is to create a path for the dragons to get to the middle of the base, because if you just place down dragons randomly, then they will most likely just go all the way around the base until they get killed. With the funnel, you take out some of the outlying buildings, so that when you place down dragons afterwards, they want to go to the center because that is where the closest buildings are.
The best spell to use with dragons is the rage spell, although the heal spell could be useful too.