With the current armour design, the armours are set up so you should always use the biggest and best available, unless you have a metal shortage.
There is little point using chainmail, it have the same total defense value against cutting (chainmail provides double cutting defense) than platemail which have a strength of double blunt defense (for some reason)...
There is little incentive to try and use counters in this game really, the best tactics I can give you is:
If the AI uses Leather Armour or unarmoured units: Use Swords (or meaby Maces for no metal cost)
If the AI uses Chain-Mails: Use Spears (or Maces again for no metal cost)
If the AI uses Plate-Mail: Use Spears, or Magical Staffs (which ignore defense completely)
In other words, I usually only bring weaponry depending on my AI's current techlevel and metal gathering rate.
Swords = Good against low armoured enemies, unless they field tons of spears.
Spears = Good against armoured enemies, does pretty well against low armoured enemies too if you ask me.
Maces = Too slow for me to bother, but they don't cost metal, so thats a plus.
Bows = Good against the AI since its dumb, you will want to bring slowing spells and meaby horses with these units.
Magical Staves = See bows, also good against heavily armoured units due to these ignoring defense and going onto a fourth and fifth defense stat.
For defense I myself prefer leather armours and maxing dodge, since dodge = king
Either that or max defense.
I usually try to have my ranged units in "No Encumbrance", maxing defense as good as I can within those parameters (leather armours until he can't curry anymore)
I usually have my tank melee units in "Max Encumbrance", since they need the defense.
I usually have my skirmish units go for dodge attribute for defense, and either low or moderate encumbrance.
Some melee units I max out damage output on, these are usually light cavalry, going low or moderate encumbrance, and stuff like the trait that gives more damage to targets that already took damage this combat.
Sincerely
~ Kongdej
