hero legend oracle stuck at level 5

You have one goal - get to the city. Don't try to kill the behemoth. You'll need a couple fast, powerful units. Cavalry Commander is ideal, especially if you have March. I put Pompey on the swordsman, and by this time you hopefully have an upgraded Terracotta Warrior. That will be enough to take the city. Go through your fortifications to the south, on the first turn they might attack the enemy units inside the barrier, but other than that they should try to avoid fighting and go straight to the city. Use forced march to help. The rest of your units and monsters are the defensive force. For the first couple turns while the behemoth is held back by the fortifications, try to take out the enemy force to the south. Remember that your mammoth is great against ships, and in general ships are weak so try to take them out before they land. Once the behemoth breaks through, just meatshield - mammoth is great for that too if you've upgraded at least a few levels, though against the thump attack the light-armored units aren't so weak either.