While playing mission 14 on the hardest difficulty, I realized that no matter how long you wait, the AI, will not be able to take any more buildings and progress towards you allowing you to build your troops to maximum strength. You also start with strong units, so can easily take the strong buildings and overrun the enemy.
Immediately take the weak building below you, once it is taken, move all troops back to your starting building. Take the fort next to the weak building next, then move all troops back to the original building. You can then use your strong troops to take the building next to it.
Use your strong troops to take the strong building the Blue troops are trying to take, and then make all your troops take the blue buildings. Swarming them with all troops, allows you to complete the mission faster.
Use your strong troops, to take the last strong building, and make your fast troops take the green’s strong troop building. Once this building is taken, just send all troops to the last building and you win 1000 gold.