Attacking

What is the most effective attack?
Short answer: A single attack, made by one type of unit that can break the defense in her weakest part.
Each troop has 1 type of attack and 3 types of defense, a speed and a farm space(FS). Troops with a high attack and speed and low defense stats are offensive units like: slingers, horsemen.
Troops with a low attack and speed and  high defense stats are defensive units like: swordsmen, archers.
Troops with balanced stats “can” be used for attack and defense like: hoplites and chariots(I'll discuss more in nukes). Because they are balanced, their attack isn’t too effective, the same with the defense, except hoplites.
How is the attack calculated?
The attack power is calculated by the sum of all attacking units. So you are attacking with 10 slingers, your attack power is 23 x 30 = 690 with distance weapons.
10 horsemen, your attack power is 55 x 10 = 550 with blunt attack.
With the defense is almost the same. If you are defending with 30 swordsmen your defense is 420 against blunt, 240 against sharp and 900 against distance weapons.
So what will happen?
The distance weapons attack interacts with distance weapons defense and the blunt attack interacts with blunt defense. As a general rule if attack>defense then all defending units die, and if attack<defense then all attacking units will die - it's that simple!
What is the most effective attack? Mixed one or a single attack?
Your goal must be to break their defense in the “weakest part”. Look your attacking with one type of attack and the defender has three types of defense. It’s obvious that you should strengthen your attack with one type attack, because if you mix your attack the defender has the advantage, just as I had written above your ATTACK interacts with ONE DEFENSE, there’s no reason to interact with all 3 types of defenses by doing a mixed attack.
Two separated attacks of 100 horsemen are worse than one attack made by 200 horsemen, the size matters!
You should dedicate each of your offensive cities with one type of attack to build nukes.
Nukes are the most powerful offensive armies in Grepolis. They are made by one type of troop and bring ~2800 of population to the battlefield.
Tips:

•  Always attacks with all your offensive units and try to attack with your the maximum capacity of your Farm.
• Specialize your city as fast as possible, if you try to do everything, it will be a mess in the future. It’s normal to attack with mix units in the beginning because you receive them as rewards in the quests. Even though, start building your nukes (slingers or horsemen) from the beginning, because once you complete the bandits camp, its disappears with the rewards;
• Always escort your fast transport ships with at least 50 Light Ships, if you lose your battle naval against birremes, your attack will be much less efficient. 
• Never attack with defensive units, this includes hoplites. Unless you’re farming and you know that there are no units defending the ghost city or inactive player.
• If the defender has many swordsmen, then attack with horsemen. Of the defender has many hoplites then attack with slingers, or ask someone to attack this guy.
• You don’t know how many catapults you have to send? 10 is a good number. They are effective with slingers, only. Remember, as catapults and slingers have the same type of attack, their power is increased.