The Stronghold is a strategy that i use sometimes. The formation is maybe not the best, but its almost invincible. The goal is to have tanks (ships with lots of HP) and some really good missile ships. Its also EXTREMELY recommended to have the missiles scattering reasearchs done. The recommended tanks are Nihelbets. For the missle ships, you can either use mall ships (they are good) or create them yourself. This is the formation:
T T T
T M T
T T T
T = Tank ships
M = Missile ships
As you can see, the tanks protect the missile ships. The reason of the scattering reasearch is that all WILL go faster. If you dont have the scattering done, the battle would last almost forever. This formation is recommended only to those who doesnt wants to lose ships. It can only reach effectivness when the fleet is full (27000 ships). Otherwise, it will not be so useful, since you can only attack 1 case per round + scattering damage. If you want, you can also costumise some good tanks with some weaponry. If so, put like 2 Razor Missile Pod at the maximum, because we want tanks. Their goal is to protect the main attacker at the middle of the fleet. Please take note that the battle would maybe last longer than usual.
NOTE from other user: Maybe stronghold is a good strategy, but the problem resides on the scattering research of the opponent (laser and missile weapons) and can destroy the missile ships leaving you with a tank, no damage hollow fleet. Also, for the best damage get Extreme counterattack so the tank ships can do damage.