
Unless your army is undead or mostly composed of mechanical / non-living creatures, maxing out Leadership is essential as it can make few of your creatures have two turns during each combat round. One of your main heroes should have the Tactics skill as it can make your army twice as powerful. Increases morale and luck of all friendly creatures. Grandmaster: +50% to Melee and Ranged Defence. Master: +40% to Melee and Ranged Defence. Expert: +30% to Melee and Ranged Defence. Advanced: +20% to Melee and Ranged Defence. Increases Melee and Ranged Defence of all friendly creatures. Grandmaster: +50% to Melee and Ranged Attack. Advanced: +20% to Melee and Ranged Attack. Increases Melee and Ranged Attack of all friendly creatures. Requires Expert Offense and Expert Defence. Requires Advanced Offense and Advanced Defence. Requires Basic Offense and Basic Defence. Increases speed and movement of all friendly creatures. I've tested the below descriptions to make sure they are accurate. There are even some errors in the in-game descriptions (for example, Master Combat description says that it is required for Grandmaster Melee, when it is clearly not). The manual describes some skills incorrectly.

Each skill, primary or secondary, has five levels of mastery.

Once the primary skill is learned, you will be able to learn all of its associated secondary skills. To learn the secondary skills, their parent primary skill must be learned first. Each primary skill has three associated secondary skills. There are 9 primary skills in Heroes of Might and Magic IV - 4 Might and 5 Magic primary skills.
