This might be a stupid question, but I ask it anyway.
Let's take it in the perspective that there's not mixed units. What unit should I use to defeat a light infantry, and which one to defeat Heavy infantry, and which one to defeatPhalanx and which one to defeat Cavalry?
You need to have more offense power than the defense power towards the unit type of the particular unit you want to defeat.
For example, take the Trojan Thorakites Defense Stats
To defeat the thorakites with Swordsmen(or any light infantry) you would need their combined strength to be atleast 381 offense power
But if you are using Phalanx units (promachos for example) then you would need their strength to be more than 500 offense power to defeat the thorakites.
The same system follows for heavy infantry and cavalry.
You are right, both Thurepheroes and Thorakites defend well against the phalanx units. All other unit types defend the same way too for example, petlast defends well against heavies and psilios against lights.
But there is one exception, the imperial archer. It has lesser defense against the heavies(I haven't cross checked the stats of imperial psilios and M Petlast though)
Thank you for the detailed explanation, Ashu 
He is absolutely correct, of course. Different unit types can counter each other quite well. Sometimes you can overwhelm a seemingly stronger force with the correct application of your troops, so be certain to scout your target and draw up your attack plans accordingly!
