The guide will be extremely useful for beginners, as well as experienced but thrifty players.

I do not guarantee that I understand exactly how the experience system works in the game, BUT my method works and I just want to share it with you.

Almost straight to the point.
Each squad (or unit, if it is a squad of one character) has its own maximum level.

There are three level caps in the game:
1) 20 lvl - Regular squads. (knights, cavalry, archers, etc.)
2) 60 lvl - Heroes. (starting faction hero, tavern heroes, dragons, etc.)
3) 100 lvl - Elite. (Mages)

With the help of points received for levels, you pump up the attributes of your squads. Accordingly, you will want to get maximum levels in a minimum amount of time to be sure to win in PVP or just make your life easier in PVE.
Units gain experience for actions performed that CAN cause damage.
The more units in the squad, the faster the level is pumped.
Accordingly, pumping up a single hero will take the longest. (especially if he is also a ranged one)

Step 1
We assemble our army from those we will pump up.
The method will not work on single units in small quantities (1-4 pieces) and ranged squads.
(I take an additional couple of pumped-up squads in order to take the necessary position in these "exercises" more easily and quickly.
We move to the location " Bolfores`l " or any other and send the army into battle using the "View" button.

Step 2
2.1. After loading, we safely skip the intro and deal with the enemy that will constantly meet you in this location.
Next, you do not need to click "Close" in the menu-hint at the top, so that the reverse timer (30 minutes) does not start, after which you will simply be returned back to the world map. Of course, the whole process will take much less time and the timer will not have time to end. (But what if you need a little more time? It's just a matter of comfort.)
2.2. Again, for convenience, you can assign your squads to numbers to make it easier to navigate between them. (Select the desired squads, hold down Ctrl + number)
2.3. The arrow indicates a convenient healing source, if problems arise, we can heal our soldiers there. (green cross)
2.4. The arrow below indicates the camp that we will use for leveling. (orange-yellow)

Step 3
3.1. Now we need to clear this marked camp (orange-yellow) from ALL units except the ENTS. (The location of the ENTs is marked with red squares)
You will have to control the squads a little so that they do not start attacking the ENTS.
3.2. Once you finish clearing, make sure that there is no one left nearby who could interfere with your leveling. Clearing the upper orange-yellow camp is NOT necessary, as well as all the red camps. We only need one. (Wolves, ogres or small enemy units sometimes move around the map. Keep an eye on this to avoid unnecessary losses.)
3.3. Then everything is simple. Select more units that we will pump, give them an aggressive stance and send them next to a living Ent (about 5-6 units of 10-20 units per unit). The units themselves will fill all the free space around them and start attacking.
3.4. When the units are pumped to the level you need, you can safely return to the world map by pressing the "World Map" button.
3.5. After exiting, you can immediately repeat the entry again, if you wish.

Interesting to know!
If you did everything correctly, the Ents will lose HP only from the first hits of each new fighter who attacked him. Having a sufficient amount of HP and resistances, they will definitely survive the influx of the first hits of a large number of your units, and then they will simply STOP wasting HP from all subsequent hits of those who are already hitting them. That is why when clearing the camp it is very important not to attack the Ents once again.

1. Ranged units in any quantity, as well as single units in small quantities, will simply kill the Ent.
2. Melee units in large numbers "break?" him and make him immortal, which allows him to level up.
3. The method works on the Ents in any location and any camp. (I showed the one that was convenient for me personally)

Step 1
The initial sequence of actions is similar to what we did earlier.
We collect several dragons in our army that we will level up and 10-15 leveled up (for greater survivability, but not necessary) units of heavy infantry or cavalry. It is also advisable to have an ice dragon or ice mage with you, it will be easier with them, but you can do without them.
We move to the location " Bolfores`l " and send the army into battle using the "View" button.

Step 2
2.1. After loading, we safely skip the intro and deal with the pack of wolves that will constantly meet you in this location.
Next, do not press "Close" in the menu-hint at the top, so that the reverse timer (30 minutes) does not start, after which it will simply return you back to the world map. (Here it will take more time)
2.2. For convenience, we assign our units to numbers to switch between them faster. (Select the units you need, hold down Ctrl + number, assign each dragon to a separate number)
2.3. The arrow indicates the healing source we need to upgrade. (green cross)

Step 3
Now we decide who exactly we will upgrade and move to the healing source.

3.1. We begin to work on the arrangement we need. All the infantry, cavalry, etc. units that you took with you are placed as close as possible to each other approximately in the center of the healing source. Next, we place the dragon that we will upgrade near our units. (In the example - a red dragon) If there is an ice dragon or ice mage, then we place it opposite the dragon that will be upgraded. (In the example - the ice dragon).
3.2. Now we use the red dragon's ability "Fire Wall" on our units in the center.
3.2.1. We immediately switch to our ice unit and prepare for the next step. We wait until the fire dragon's ability is fully used. Visually, this is easy to determine by the spread of fire and the rotation of the dragon's head. The ability is always used in three "sectors" in front of the dragon, namely to the left of the dragon's head, in front of the dragon's head (center) and to the right of the dragon's head.
3.3. Now we use the ice dragon's ability "Ice Wall" on our units in the center. (This will put out the fire, will not allow your units to scatter from the fire or burn completely) After the units are defrosted and fully healed, we repeat the arrangement again (if necessary) and the arson - freeze procedure.

Interesting to know!
1. If you did everything correctly, the dragon will receive a huge amount of experience, and all the units will survive and heal.
2. If you perform the procedure without ice units, then immediately after setting the fire, simply select all the units in the center (infantry, cavalry, etc.) and move them away from the fire. Wait until the fire goes out, heal the units and repeat all the actions with the arrangement and setting the fire.
3. The most difficult thing to do with this method is to level up the Green Dragon. It gets almost no experience, since it has a weak "Fire Ball" ability.
4. If you level up the Hydra using this method (instead of fire, just use the "Acid" ability), then you do not need to freeze your units. According to my observations, the Hydra's poison does more damage than fire at the point, so you need to monitor the health of the units and in a critical situation, move them away from the poisonous puddle. Hydra creates three poisonous puddles, and dragon fire works in much the same way, only without turning the head.

That's it, congratulations!
Now you have mastered my method of upgrading armies in the game.
Using the methods shown, you will upgrade your units to the maximum level much faster than in a regular battle, thereby becoming many times stronger at any stage of the game. And the gold saved on training is better spent on developing your city. Good luck!

Of course, I doubt that I will help many people with this guide due to the low popularity of the game.
At the end, I will add for especially generous people, there is no need to like, dislike, reward the guide and add to favorites, it is not worth it and does not change anything. Just take it and use it.

Thank you for your time and attention.