Just for the sake of understanding:
On the browser, open the blue "i" icon card of a Cerberus in your barracks army camp. You'll see that "Best against" and the green "Light Ranged" and purple "Mage Units." Point the mouse at the green bar and it should pop up a tooltip. This tells you exactly what bonuses the Cerberus gets against Light Ranged. What you see will vary depending on how far advanced your city is, but for mine, I see just:
Dealt against Light Ranged Unit: +30%
Contrast with their rating for mage units:
Dealt against Mage Units: +70%
Taken from Mage Units: -40%
You can see they're a lot better against mages. They have a defense against mages (mage attacks do 40% less damage than normal) but they don't have any defenses against Light Ranged.
The way unit classifications work is every unit has these sorts of bonuses against exactly two other unit types. Those two types are the ones the unit is said to be "best against" but the actual bonuses vary. These two types are also what form the "pentagram" because all you're doing, basically, is drawing lines from units to the two that they have bonuses against. Keep in mind no unit type has bonuses against more (or fewer) than two unit types. It forms a very definite pattern.
One thing that I have noticed about the AI is it appears to always try to move the farthest distance it can that still puts it into range of whatever target it has decided to hit. I don't know why it does this. You can see it in action when playing manual: If you are issuing a command to one of your units, and you point the mouse at an enemy that your unit is not in range to hit, you will be shown the movement path the game is proposing for you, and the place where the AI tries to move the unit is almost always insane. If you are going to fight in manual I suggest always being careful to move your units yourself and not let the computer decide their movements. (Your units won't move if they are already within range, so sometimes you may have to move them bit by bit first, then click the target once you're within range.)
By the way, Sorceress has attack range of 3 and movement range of 4.
You can point at the icons to see the text saying what the icon stands for.
I have Orc Strategists in all of my cities already by crafting a Grounds of the Orc Strategist from the Magic Academy. But, they are only 1 star. Still useful though.