The combat simulator allows you to conduct offensive and defensive analysis; it can simulate the most likely outcome of a battle based on the info you provide it with.
There are a wide array of factors taken into account during battles, so the combat simulator is not going to be 100% accurate but it will give you a likely scenario.
If you know how to read into the information you are provided with by the simulator, you can make valid assumptions then on what you need to do to secure your victory.
You might consider: increasing the number of troops, increasing Smithy levels for troops, adding specific units to counter enemy units and so on.
There are thousands of variations based on what tribe(s) you and your allies are and what troops are being used on one side and your enemy (or enemies) tribe and troops on the other.