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A Knight can handle a lot against the undead, but against Markel, you need someone stronger – for example:
1. Godrick – well, I don’t know, he has two perks at the beginning, Light and Leadership, which are harder to level up, although Prayer does its job... if you collect a Lion set, you might get lucky... but overall, he's a decent hero (you can easily level him up through Combat Arts – the best choice... and then, under your supervision, the hero will destroy the entire army).
(If you think about it, it's possible that there will be a week of some kind of undead, like vampires, and then it will be tough).
2. Frida – she's also not bad, her ability is simply made for the late game... so if Markel finds 30 undead + his 30, the total might be enough to kill someone with one hit, and that's when we'll need Frida with her Divine Retribution...
...And also, if the map has the Tear of Ashera, it's better to find it quickly, and you're good to go...
3. Dugald, Laslo – they're decent guys because there's a guarantee that they won't be defeated at the beginning, and when you have more archers or squires, you train them instead of paladins... and it's desirable to have Eternal Light...
4. Axel – he's just a great find for the late game, training paladins forever, and that's the end for any Markel – it's better to take paladins... Knights and Isabel hit hard, but there's a risk that Berserk will be applied...
...Conclusion:
If we play for two and a half months, there will be enough time to buy artifacts: it's desirable to find Academy or League castles and collect a set: Lion or Dwarven, on a large map, anything is possible...
...Markel will try to overwhelm with numbers and use magic, Darkness, or Summoning + something from other skills: Defense or Education, so we go through Combat Arts and beat the undead.