I've never been able to beat the computer, even on the hardest level, but I can reload and replay many times until I'm happy with the outcome, with minimal or no losses. ))))
Right now, I'm enjoying the Knight faction. I always start with upgraded archers and divide them into 7 squads. The enemy attacks and runs away. I shoot them down. If they attack without mana, they usually retreat with almost no losses on my side. The heroes decide the outcome: Sir Mullich due to his +2 speed bonus, Oren due to his 100% ranged attack bonus at level 20, and Valeska due to her charged archers. But I also really like Touris with her tactics and charged champions. Sorcha and her powerful knights with their devastating double strike are also good, and the specialist for griffons with armor works wonders (a deadly combination: several archangels and upgraded griffons, which respond to everything, and thanks to "Armor" and increased defense/attack stats, they turn into terminators, tanks, and killing machines in sieges).

Another great tactic is "hit and run." For example, 2 Phoenixes or 1, 4 Efreet or Golden, Black Dragons ++++ ARMAGEDDON... it's just devastating. I was able to destroy 15 archangels by the end of the XL map. The enemy was building up an army for 4-5 months in a confined area, which sometimes happens on random maps. I flew in through a dimension door or by flight and solved everything with one Armageddon. Sometimes there are setbacks... the enemy has monsters with immunity, summons fire elementals, or has artifacts that prevent spellcasting or good magic resistance, sometimes War Chains, but as a rule, almost everyone falls. Even if I don't manage to destroy everyone, having 2-3 "HIT-and-RUN" units can seriously weaken the enemy's army, and then I finish them off with my main hero. Another important thing is to send heroes with fast monsters, with speeds of 6-7-8-9, for scouting, and divide them into 1 unit per 6 slots, so they can cast magic 1-2 times and then run away to the tavern.

Added 11 minutes ago

Yes, to avoid unpleasant situations like in the first post, you need to try to develop quickly from the start and attack the enemy first, when he is not yet so strong, and his mana reserve does not allow him to "suffocate" you, or he does not yet have powerful high-level spells.
Just yesterday, before going to sleep, I had a similar situation: I attacked the enemy several times, and each time he defeated me, and he "won" precisely by summoning a bunch of stacks of elementals; if it weren't for the elementals, I probably would have defeated him; he has more "meat," but I have more mana.

I can't crush him in the first month because the map is very large, there are 7 enemies, and I also start with 0 in all indicators, because otherwise it's not interesting. As a result, I fall behind in development by a month without the initial 30,000 and resources, but then I catch up. It takes 27,500 to build up to the capital, so I only collect them by the end of the 2nd-3rd week. And sometimes I'm completely surrounded by archers or a horde (crowd) of ranged units and just wait for my troops to accumulate, but then I go and crush everyone, even though I have 2-3 times fewer troops