Even though I wasn't playing on the highest difficulty, I noticed that not everyone managed to build a strong (full) army. So, I defeated two tough opponents and passed the turn. One of those tough opponents defeated a weak one and attacked me, while the others fought amongst themselves, leaving only me and one other tough opponent.
In short, on a medium difficulty level, you can defeat all the AI opponents; the main thing is to act quickly before they start fighting each other.
As for development, it's optimal to plan your strategy so that you have enough money and can build a strong army.