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the ideal option: a dread knight (because all skills are free, and there's room to develop, dark magic doesn't count, as even the weakest hero will level it up).
1. Develop dark magic, take the "ensnare" spell, and attack the enemy with their own troops. Essentially, it's a legalized cheat, considering that magic levels up the fastest, and the spell's power becomes too strong for the opponent to counter for at least the next two turns. As for skills, it's probably best to take "master of curses/mind/pain."
2. Education: the ability to develop a hero faster than the opponent is always useful. Again, see point one about magic and "ensnare." Skills: magic attraction (given that their knowledge isn't phenomenal, it's necessary), graduate + magical reward.
3. Attack: initially weak, so bonuses won't hurt. Take archery, battle frenzy + haste (cold steel, magic resistance, and chaos magic are not available).
4. Defense: undead troops don't have great health or defense, so level it up + grave chill + resilience (or evasion) + reflection.
5. Logistics (or spell power, but in that case, magic becomes incredibly and pointlessly powerful) - because arriving at a location before the end of the week or before the opponent arrives can be very important for speeding up the process of building cities (if you don't get the ore in time, you lose the opportunity to build, which means you lose a week's worth of creature production) and capturing them, and it's also useful to retreat for reinforcements.
Of course, all of this is accompanied by developing necromancy.
In Lucretia's case, the logistics issue is not relevant because the spell power slot is already taken. It's a pity, because a bonus to vampires (the most resilient units, and therefore their attack power doesn't decrease from turn to turn) would be useful