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If playing on a large map (for example: "Rise to Power"), you can level up a hero to level 25-27 (correspondingly, their knowledge will be around 25-30, especially if you take Education), then you can do the following:
Gremlins: Health(8), initiative(around 30), and for example magic defense;
Gargoyles: Health, initiative and speed(3);
Golems: speed(3+2), initiative(30+2), if playing as Razzak (the creator of golems) and upgrading "Golem March", then the golems will be absolutely unbearable, and also luck(4);
Mages: initiative, luck and magic resistance;
Djinn: speed, initiative, health/armor;
Rakshasas: speed, initiative and attack
Titans: initiative, luck and attack.
Magic resistance helps a lot if playing against Warlock; you can equip the entire army with it!