Summary: Essentially, you get 19 to 50 mana per turn early game to spam heroism, hellhounds, and earth lore to take over an early empire, then equip heroes with cheap items.
1. Retorts, Books, and Spells:
Take 4, 2, 2 in order to get Archmage, Runemaster, and Artificer. The best by far is 4 life, 2 chaos, and 2 nature, but you can mix it up for fun. Reason it's "best" is you can get heroism/healing/just cause/true light, hell hounds, and earth lore.
Any race works, but I'd recommend a builder race. You won't need too many normal units. Halflings obviously the most OP race, but humans, orcs, elves, and nomads are also good.
3. The Start
Switch your town to building a granary, and switch to full mana in the magic tab. Go to your spells and start crafting a 38mana sword (normally 150).
Once your sword completes, melt it for 75 mana. Now go back to the magic tab and put all points into skill. You will no longer need to generate mana through the magic tab. Start building another sword, typically you can start 50g swords that give you 100 mana (50 net), and as you build a sword per turn, you can increase the price per turn and mana per turn as well.
4. Early Mana use
If you took at least 2 points in life, you better have taken heroism. With your early mana pool, if your swordsman comes to a town of 3 or fewer swordsman/spearmen, just go in and 1v3 using heroism. Your spearman can sometimes take 3, but it's risky, and it's better to just do 2 or fewer or keep scouting.
If you took at least 2 points in chaos, you can start building hellhounds once you amass at least 200 mana. Remember that each hellhound can take 2-3 normal units with micro, so 3 hellhounds can take a town of 9 normal units (non-ranged).
If you took at least 2 points in nature, use earth lore to help you scout.
5. Heroes and Items
As your fame grows, hopefully a hero will come by. Never reject the first hero. Just take it. It's very rare for heroes to come along on their own. Melee heroes are the most common, and a good first buy is a 3-attack, 3-defense sword with flaming and one other attribute. Your mana spam can also get you a cheap shield with 4 defense/4 resist/3movement for 300 mana, leaving room for another attribute like blessed or resist elements.
As your towns build up, put a priority on getting the "Summon Hero" spell. 6 fully equipped heroes will take out anything of course, but even a single hero with 3 items can take out any non-wizard city.