Table for attack bonus

The best use of this table is knowing which monsters you have an attack bonus/penalty against with your specific troop type.

The chart is talking from an attack perspective. If your troops are using piercing attacks they will deal 75% damage against medium armor, 100% against light, and 150% against heavy armor. Don't worry about attacking fortified armor with anything other than siege attacks, or you will just lose a ton of troops. Also, don't worry about the armor of your troops relative to the attacks of the monsters you are attacking. For the most part you are gonna just match your attack up with the monsters' armor and send in speed bumps.
For example, if you are attacking goblins you will want to use axe throwers because their brunt attacks do 150% against the goblins' medium armor. But to prevent the goblins from getting to your axe throwers, you need to send in speed bumps (units that will die, but will allow your axe throwers to eliminate the goblins). 1 militia (and 1 of any other unit you want to use) should serve the purpose.