When attacking, it is your power versus your targets hospital capacity and resources. Before you deploy your troops make sure you send out a radio-controlled plane to recce the enemy you plan on attacking so you have some idea what it is you are up against. When you do attack a good tactic is to send three times the number of troops you are attacking just in case the enemy has other players soldiers in their base defending. Doing this will also help reduce the amount of loses you will have.
When you teleport to attack an enemy you need to make sure you set yourself as close to them possible as this allows you to attack much faster and complete your goal in a quicker time. Another strategy is to consider an alliance with a friend who can put up a shield and send all their troops to your base where both yours and his troops attack the same target for a lot less loss of troops.