Stronghold – “Of Worms and Demons” [Campaign 6, Mission 4]

Probably more of a warrior than a mage, although I use both reinforcement with counterattack and weakening with regeneration! I'm developing him to the fullest.

Sandor is a warrior by default! But you can change the hero to a mage with a license.
You're developing him towards Blood?? Hmm... by the 4th mission, you should have already fully become a blood tyrant! But still, for a warrior, Tears are better! A MAGE, definitely, BLOOD!

I've already built and upgraded Cyclops. I closed 1 portal, 3 are already open... I only have one Castle so far. I feel like I made a mistake by bringing Akamas back from the seas.

Stop! You've already upgraded Cyclops in one Castle??? That's good, but it's early! It's better to throw all your forces into conquering a second Castle!!

The most destructive enemy, causing losses to my army, turned out to be Tainan.

That's right! You need Regeneration and Reinforcement against him! Close the portals - it's a must!!!!!!! It's better to buy a new hero, upgrade logistics and seafaring, and send him to the seas! He will then sail and CLOSE THE ISLAND PORTALS!!

Added 6 minutes ago

My tactic on this map. I'm playing on hard difficulty. Sandor is a Tears warrior.
I give the Centaurs and Harpies to Airini, and all the rest to Sandor. I send them in different directions to capture everything they can handle, and it turns out that almost everything is within their reach.

Airini is a hero who specializes in "Tears," as I understand? Did she already have upgrades? Or did you continue to develop her and upgrade her?

Airini is a mage in battle.
First turn: Earth Elementals (the entire battle in defense) to the enemy archers, and Centaurs and Harpies bombard the enemy.
For Sandor:
Reinforcement on the Jaguars, command to wait, goblins and Cyclops bombard (if they are already there), Dreamcatchers cast a cloud (it's called Sky and Earth or something), when the enemies move back, we rush in with the Jaguars, trying to engage as many enemies as possible.

Great! I immediately throw the Jaguars into battle, I just move them to the center of the map and cast "Counterattack." I have the passive skill "First Counterattack" upgraded. Don't be afraid that they will die. Second turn - "Weakening" and forward! Dreamcatchers are my favorites. First turn, you're right, magic on everyone, second - finish them off!
Sometimes, if the enemy army is strong, I use Reinforcement on the Jaguars. But then they won't attack first.