See my reply to a post below...
After I replied to the post I thought that this would make a good blog...so here it is.
When you are looking at designs matched to your Town Hall level you will notice that the towers and Hero Bases are mostly concentrated close together. This concentration of buildings in turn allows the user to maximize firepower in the direction that the monsters are coming from, with a minimum loss of overlap in the buildings' engagement radius'
It's this concentrated fire that takes the HBM waves down quickly
Since different troop typess travel at different speeds and go after different building types (destructive troops bee-line to towers) the HBM wave will break up and lose cohesiveness thereby ensuring that the different troop types will arrive at your core area's attack buildings at different times. This fact combined with the attack building's concentration allows you to destroy the wave in a piecemeal fashion (defeat in detail). It also ensures that any one of those same buildings does not receive the full attack power of the entire HBM wave all at once which is good from a defensive standpoint. This point helps explain why so many base designs have entrances.
Secondly, in "proper" base design, you will notice that walls and secondary buildings are placed so that transit times of the HBM monsters is maximized before they can reach the core protected area containing your attack buildings. This allows for more ranged attacks by the towers on the incoming monsters.
Third, again in "proper" base design, you want to minimize the amount of the Hero Base's engagement radius that is outside of the base's walls. This helps to minimize any "wandering" your hero might do. Any hero(s) that acts alone and engages the HBM waves by themselves is much more likely to die than if they act in concert with the rest of the heroes and towers.
For me this would usually happen during the first two waves of a HBM level when the monsters were busy attacking the buildings outside of my core area. And having all your heroes alive and towers intact at the start of the third wave greatly increases your chances of completing the level.
So concentrate your firepower, maximize HBM monster travel time, and don't let your heroes act alone... and you see positive results.