I am playing the campaign and I"m a bit in. I've just met Ul-Dagan and have unlocked T4 units. However, Beleth invaded me! No problem I thought since he's waaaaayy weaker than me. Except by about turn 30 or so, he's knocking on my front door with a massive army that I can't kill.

What is the best and quickest way to build up enough that I can face an enemy AI? He has a warrior that can kill my warrior, and his army, my scout and his army, and STILL kill the guards I have on my provinces. For example, I'm doing my exploration thing, and run into some hefty neutral provinces, like 10 centaurs vs my level 10 warrior. He just marches through them as if they were nothing.

I've noticed this throughout the game, that the AI somehow gets enough gold and XP that they have 3 champions, each that can beat my own champion. What is a good defensive strategy to stop this from happening? Should I be using a different hero?

I've pretty much given up playing the game at this point because I've been slogging through it, then bam, this one enemy master has the ability to crush me on my own home shard and there isn't anything I can do about it. At this point I'd even be willing to just cheat to see the end of the game I"ve put so much time into.

Any advice?

1. You have extreme advanatage. You can see whole map.
Check, is there a fast way to enemy - free settlements, hobbits, nomads. And check for centaurs near enemy - AI can ally nonhumans with some chance, he doesn't need quest.
If you make free road for AI to your demesne - it will forget about exploring, building, and will go straight to you. Kill him, while he low level. and again, and again until you can take on his guards.
2. You have some free items and units. Use them.
3. Don't explore.
4. Centaurs really easy for warrior - chestpiece (+3/+3), armour mastery (+3/+3) + magic armor (+3/+3) = 9 defense(+2 from forest). Add shield, if you not sure (you can swap it later). Centaurs have 7 base ranged damage, will barely scratch you, and while they shoot, they loose stamina, so even high level centaurs would hit point where they can't damage you from range or even melee. When they spend all arrows, move on hill(+def,+counterattack) or in corner(easier to regain stamina). 2 Magic armors +2 Astral energies probably will do the trick.

I can see the whole map. I can see him march towards me as he takes down T2 and T3 neutral provinces. I try to attack the same provinces and get wasted. He somehow rolls over them, and still shows up at my doorstep with full armies. I look at the provinces I have and I have WAY more gold income than he does, yet I can't afford that many guards or armies!

I usually do explore because that is how I gain levels for my hero. Should I just attack and ressurect my hero? Is that the best way to go about it, even at level 1?

My free items are crap. The starting warrior items are just as good.

Any other advice?

Theorycraft isn't very helpful, imo.
Share map0.cmp and sav0.cmp, preferable on first turn. (If you have more than 1 profile, it could be not 0 in name)

Theorycraft - suggesting what to do in theory. Basically, I don't know what sites you do have in demesne, surrounding, strategic resources, even possibility to ally someone.

Second part - add to archive these 2 files and share on some service. I prefer dropbox, they don't limit speed.

"Theorycraft" is us discussing the situation in abstract terms. We can give general advice, but we have to guess about a lot of the details.

He's saying to upload a copy of your save game somewhere (dropbox, google drive, etc.). That way people can take a look at the exact situation.

Oh cool! Well I took his advice and expanded much more quickly. I ended up winning very fast. It was tough, but it worked. I'm just getting burned out on the game though. I want to see how it ends rather than have to take forever to burn through each shard. Is there a way to just kind of cheat through at this point? I've put in so many turns I"m getting bored:(

Just take a break and continue later, the game is way to good not to come back to it every few months.

I just tried out the Centaur strategy with my level 5 Warrior who had 3/3 Defense from Armor Mastery, 2/2 from Simpe Chainmail and ofc 2 Magic Armor spells. Other gears gave 16 HP and +1 Parry. Weapon Mastery 2 and Constitution 1. Sadly weapon is only a 5/4 2 Handed Sword.

He ran into 8 Centaurs with avg level of 8.5. He managed to kill 2 and almost a 3rd before being taken down :(

Got to find fewer to level up a bit more and some better weapons. I really love First Strike + Hex weapons for him as I mostly solo him till he is way high up.

Taking a break is a good idea. I want to know how the game ends, I just get tired of repeatedly going through and taking these shards. Then they invade me and I defend, and I am actually not ahead at all. It doesn't seem to penalize them for a failed invasion at all!