Haven tactics.

P.S. Against a Gnome on level 21, Haven doesn't stand a chance either.

I only play against the AI on Veteran difficulty, so it's hard to judge about "no chance." I'm sure (to say the least) that this is not the case.
So, I played against the AI 1 on 1 on the "Lands of Outcasts" map. Humans against Gnomes (I played as Humans). The Gnomes had no chance)))
On this map, there are simply a lot of resources, and every week I recruited the maximum number of Isabel Knights, dedicating all the monks and part of the swordsmen to this.
Development ended at level 19, with the hero Axel.
I had several times more Isabel Knights than the Gnomes had Yarls. Plus, leadership was maxed out with Inspiration and Empathy taken. You could say that the Isabel Knights were the only ones moving (they moved very often).

So, Humans have a chance against absolutely any faction. It all depends on the map. If there is an opportunity to train at least 10 cavalry units per week, then this is very serious.

And I wouldn't take Reflection against a Necromancer. Seriously. It's not needed against the undead. You need to prioritize maxing out leadership and definitely taking Inspiration.

Now I've reconsidered my build for Humans)
I really liked this one:

Axel, level 21 (against a Gnome):

Developed Counter Strike - Watchful - Experienced War Leader
Expert Leadership - Rallying - Inspiration - Empathy
Expert Attack - Tactics - Battle Fury - Retribution
Developed Logistics
Expert Defense
Basic Light Magic - Righteous Fury

Or, if there is a Divine Wrath or Resurrection spell in the guild of magic, then we don't take Battle Fury and Retribution, but max out Light Magic. Or, we don't max out Light Magic at all, but max out Logistics and Counter Strike.
And there are also some minor adjustments. The main thing is to max out leadership, attack, and definitely take Tactics.
However, there is one caveat... The Gnome can develop into Chaos and get Armageddon. Then you will have to take Reflection.

Against a Necromancer
Axel (level 21)

Developed Counter Strike - Watchful
Expert Leadership - Rallying - Inspiration
Expert Light Magic - Blessing - Righteous Fury
Expert Attack - Tactics - Archery
Expert Defense
Developed Logistics

Again, there are options. Don't take Righteous Fury, but take Expert Counter Strike or, say, Prayer (battle spirit will be at least +5).

Added after 3 hours 58 minutes
By the way, regarding tactics for Humans.
I probably won't say anything new, but I tried it myself and was very pleased with this development method for this castle.
In addition to the main hero, you should quickly level up a secondary hero and take all the skills necessary for retraining, i.e., Counter Strike to Expert level and Experienced War Leader.
Then everything is simple. The main hero (preferably Axel) travels the map, captures mines, and levels up. And the secondary hero, level 6-7, sits in the castle and converts swordsmen and monks into cavalry.
The most economical solution is to convert all 7 monks into cavalry. But then, we lose a valuable ranged unit.
A more expensive option is to convert 20 shieldbearers into 10 cavalry. This is more expensive, but then the Inquisitors/Adept remain with us.
You can, of course, convert 7 monks into cavalry + 6 cavalry from shieldbearers, and next week 7 + 7.
It all depends on your preferences and the amount of resources.
Ideally, make Elaine the secondary hero and have her gather all the peasants, and also take the skill from the Leadership branch - Estates.