Ok so i finally decided to give the campaign a go after i realized the save system works differently to the earlier version of the game where you would lose all campaign progress on game crash.

got to about the third or fourth shard and encountered the second master belious or something.

Started off much the same as the previous shards - sticking with tiny so far.

then it went bad fast, i mean before i got any garrison available at all, this guy takes all the land around me and somehow manages to put super powerful guards on them.
All i have is the one territory where my hero is farming money from locations. The enemy hero wont attack because my hero is more powerful. so i wait for the enemy hero to siege my castle and then i kill him.

rinse and repeat about 4 times. I think i will eventually get strong enough to take back the territories, and he will stay weak because all he is doing is reviving, running and getting killed. Also, he has to pay wages for all those garrisons, so, i think i am good, just a matter of time rite? WRONG

he comes around the fifth time and besieges my castle again, i move to kill again, but no, somehow he has multiple skeleton and zombie raise spells now.... where the F did he get all those spells all of a sudden when all he has been doing is running and dying??????

Ok so that is that, game over. Why did i waste my time on this? back to sandbox.

Update;
I decided to re-run the mission - same shard, same opponent.
I decided to Beeline for the adventurers guard and take the territory where the enemy hero came around the first time and block his path.

I did not expect that to work because i have seen how the enemy hero can magically defeat territory guards that my hero cannot touch, yet my hero is more powerful 1 on 1 verses the enemy hero that did that.

But it did work! The enemy just sat there and waited for me to be ready to win, and when i did come, his hero was super weak and did not have all those raise spells that he had the first time around. (I will note that this time the enemy chose a warrior hero, whereas last time he chose an archer hero)

I am guessing you have to not allow the enemy to grab a heap of territories or they will steamroll you.

This is one of my frustrations with the campaign. It generally rewards fast and early expansion before your opponent(s) can get established. My personal playing preference is much slower but also there's so much to see in the game that going for the quick win it feels I'm missing out. I also like to try different heroes and units but take to long exploring and I find I reach a stalemate where I can't get past the incredibly high level guards.

The Age of Wonders 3 campaign was structured in a similar way and equally frustrating when there's one main way of winning a scenario.

I.AI doesn't have to pay upkeep for guards

II.Every Master has some Free Stuff(guards inculded) he starts with and most have a alliance with one of the neutral races in addition. MoBW doesn't explain it, as far as i remember, but at the old Eador it was explained in a Way that every master has some special powers and Your(As the Player) Power is to rewind time(save/reload)

III.In Theory there is no need to rush as long as you can bet his Hero(s), you build up your army, level your Hero, get Equipment and then crush him. The downside is that the AI will place his strongest (unlimited)guard in EVERY Province=Monsterguard in every province for later shards and that can be a major grind.
But most the time not losing a hero and losing as few Units as possible is the path to victory.

IV.For Player vs AI most guards are useless, the weaker ones are free EP and you can't afford placing more then a few of the stronger ones...if at all. Builing a Fort, puting a T1 Units inside works much better most the time.

V.Ai with New Spells. Every Master start with a special setup and then advances the same Way you do, they can conquer shards during the Astralturn and they build up their demese during the War for theShard. Beleth as a Necromancer is likly to start with knowledge of Magicbuildings esp. Necromany.

Figures! lol

Since that shard i did not really have any further problems so far in the campaign (other than the constant access violation errors).

I am on my 19th shard!

There is generally enough astral energy to bring a half decent hero to the next shard, which is really a game changer compared to struggling to start with a new hero every shard.

I got a good system going of rotating between warrior hero and ranger hero... whichever was brought from the previous shard will be too experienced by the end of the new shard and has to be left behind. whichever is the new hero gets a certain amount of experience, but not too much experience, and will be brought to the next shard.

The point is, you have to avoid starting a new shard with a new hero because they are SOOOO useless it is not even funny.
The hero with a bit of experience makes the WORLD of difference believe me, and he quickly finds some decent equipment that makes your second hero just barely useful so he can actually find some battles he can win and start getting experience.