Go to main menu and click world builder this is were I was able to make as many units as I wanted and got rid of as many enimies I wanted.
Go near a city then go to world builder and should come up with a bow in the top right and show every type of battle unit choose which you ant then go on the map and left or right click and it should show up on the map and the same ver the enemy units but they will dissapear.
There is the way to get a certain unit it have every promotion so if you wanna fight the enemys then ull have a very small chance of losing
Every building in your cities is possible due to world builder. Plus every skill such as if you just started you can get the very last one and if you want every other.
If your having problems taking over an enemy city these tips should help. Make sure first that your army is as many guys or more than the population of their enemy city. Now you got to take away there defenses befor seiging it. Usually a well guarded city has +60 to +80 defense depending on it's size. So bring a good amount of artillery (catapult, cannon, artillery).
You should also bring units that have fortification abilities, each turn a unit is fortified it receives +5 on defense. Usually the first enemy city I choose to take over late in the game is either one next to a hill (+25 defense), a forest or jungle (+50 defense) or both (+75 defense). Once there defense is at zero use your artillery again(preferably ones with city raider) and use collateral damage to shock there units. Now you can use your fast units if you got them (knight, cavalry, tank, etc.), attack once with each unit and you should still have movement left with all your fast guys.
If you successfully killed all there guys in one turn you should be able to move all of your guys, (minus the artillery if there still alive) take the city, and quall the resistance. If they still have guys left after your first sweep regroup and repeat second turn. Make sure that you don't use all your guys in the attack, you want some units fortified protecting your army. If you follow my logic your armies shouldn't get slaughterd in battle. In war think about your worst case scenario and be prepared for it and you won't loose.