Maybe Trainer and other army-wide bonuses, such as Created Units and Monster % Bonus XP, Dodge, Accuracy, Attack, and Defending Defense only choose the highest single bonus from any troop to add to the army. So Trainer highest experience bonus for troops (which include henchmen) would be 60%, and the bonus applied to all other armies for Dodge, Accuracy, Attack, and Defending Defense would be the one from the hero/henchmen with the highest level. This would mean the highest dodge (and/or other stats) would be from heroes/henchmen with individual unit artifact bonuses, and the army wide bonuses would only use the highest single bonus.
Max bonus would be:
Trainer 3: +45% XP to Created Units or Monsters in Army
Defender 3: +30 Defense while Defending to Entire Army
Guardian 3: +12 Dodge to Entire Army
Tactician 3: +3 Initiative to Entire Army
Leadership 3: +30 Accuracy to Entire Army
Shieldwall: +5 Defense to Entire Army (if no units/champs have it, no bonus)
Healer (extra healing, I'm not sure how much healer heals per turn, whether it's +1HP, or +1HP per army member, or what) (if no units/champs have it, no bonus) - NOTE: could exempt this one, although for simplicity's sake possibly boost the healing of this trait (if not high enough yet) and only use it once, even if there are multiple healers in the army.
Individual or artifact bonuses could still add to this for the individual unit, but the max of each of the "General-type" bonuses listed here would be the highest skill for each of these in the army (and that doesn't have to be the leader)
Note the General trait (which leaders can have) would stack with Trainer 3, since it is a different trait.