Regarding the AI's behavior.

Experience will come with time. You will start acting more aggressively, be less afraid to lose troops on neutral territories, and at the same time, lose fewer. You will reclaim resources and build faster. And you will win.
You can also write who you are playing as, and who the bot is playing as. And what map size you choose. Then I can give specific advice (ideally, if I also know your play style, day by day).

I played the fifth game very little, and Heroes in general. I don't really have a style; I usually build for troop production + upgrades for the daily gold bonus. I'm currently ignoring the tavern and forge (probably a mistake).
I've been playing as the Dwarves, setting the bot to random, and choosing a large map size.
Regarding experience... I don't see any ways to gain it other than reading guides/tactics.
And as I already said, I can't get very far because of the bot's attacks.
I guess I'm doing something wrong %)))

Added 1 minute ago
With the computer, unfortunately, I can't have large-scale battles