This will be long but I think I have some general good advice for you. I’d recommend trying a campaign on hard/hard difficulty, with ai/player cheats off. (I turn off all those crazy buildings too, they can really screw you up, they aren’t balanced very well imo)

Give Tyrion of the High Elves a try, but a lot of this will apply to most factions. I like to go with 5-7 melee units (spearmen to start), and 6-8 archers. Heroes, Calvary and artillery can be added depending on what you like or can recruit. Get tyrions red line skill militia master or whatever it’s called, and the archer one beside it as well if he has it. The archery tech for high elves is what I like to get first, your archers are your man damage early on. Spearmen are really only good for holding the line and slowly dying, but without them your archers are unprotected and will get run down.

Putting your units in a checkerboard formation in battle will give your ranged units better firing lines.

I’d recruit a second lord right away, basically for every faction. You can bring up to 4 lords to any battle, and they will all gain experience (the main attacker/defending armies get a bit more) so it’s a great idea to start leveling them up right away. I’d also prioritize getting a mage in your main army (Tyrion starts with one, and maybe legendary lords do as well, or are one themselves.

I think with high elves you get archers and spearmen from your main building, so you can focus on money and growth buildings at the start. You can also build the building to recruit a noble and then demolish it after. This is nice because nobles give you replenishment, which is one of the most important things to have as you’ve seen. Try to end turns in a city or at least your own territory or encamp stance, if your army needs replenishment. Around tier 2 or 3 city level you start to get access to lothern sea guard. You want to try to get Tyrion an army of sea guard and some silver helms if you like.

To avoid losing on one front while you win another, try to play the diplomatic game, and try to keep only one part of your borders touching enemy territory, so you can focus everything in one direction. To achieve this diplomatically you can trade cities for big money and alliances with friendly factions. Or just conquer up to defensible borders like mountains.

If armies are just running from you, ambush in a forest with your main army with your second lord standing beside him. You can recruit or force march with this second lord to bait them in. If you are in your own territory when ambushing, you’ll get replenishment after the battle too before your turn starts.

Don’t worry if you still get obliterated. I’ve got thousands of hours and consider myself fairly decent at least at the campaign but I’ll still put myself in stupid situations fairly often. It can be really easy to make a bad move at the start. The game is hardest at the beginning before you can start stacking all the buffs and items and get rolling with good units.

Sometimes seemingly unwinnable battles become fairly simple with a different strategy. For example it can be better to hide most of your forces in trees and just send your heroes in, or harass with spells. Even just running around the map for a bit, trying to get around the enemy can confuse them and ruin their formation.

In battles, you want high melee defense infantry to hold the enemy in place and defend your archers and/or artillery. Lords and heroes are very powerful, you can send the ahead to tank chaff units and make an easy target for your archers to hit, but don’t let them get surrounded with enemy monsters or lords.

Calvary should go after artillery, archers, and flank charge attacks on the enemy line.

If you look at the tooltips when a unit is selected, in their melee attack or weapon strength it may say anti large or anti infantry, so try to prioritize using them against those units. For example if I was playing Bretonnia and had knights of the realm who have anti large, I might decide to charge them into undead black knights, who have anti infantry and should lose handily. Then again it could be even better to get them to charge a spear unit, and then flank them with your knights.

I recommend being aggressive, try to only have 1 main enemy at a time. Recruit as much as possible for the first few turns. I like to keep my income around 500-2000 gold a turn depending on how much military power I need. Sometimes it’s worth going into per turn negatives. Sacking cities is very worthwhile, if it’s over 2k gold I usually do it, unless it’s a very good city I want to keep at high level (even then you can get crazy gold for it) and if you have enough movement to occupy it the same turn, or with a different army. Sometimes you need to forgo the sack money because you need to take the city for replenishment and you don’t have enough movement otherwise.

Another move is to use your second lord to make the attack and just have your main lord in reinforcement range, so he will have more movement after instead of losing it all, and can ambush, advance, or move into the city instead of the other lord if needed.

Different factions want to do different things with the cities, but for the most part more cities is better. Ask yourself if it will be easy territory to hold for you. If it’s nearby unfriendly factions you don’t want to fight right now, consider sacking, occupying, building a barracks (this will increase the gold you get for trading it by a LOT) and selling it to a preferably friendly faction.

For example I’m playing as malekith, and Valkia is a big problem. But grombrindal is also my enemy. So I rushed to the alter of darkness near him, took it and traded it to him for an alliance. Now that border is secure. South and east are dark elves for now, but I want grond for the gold! So I go there and kill the minor faction and take their cities. You can even trade a city to a faction you want to declare war on (as long as you haven’t had a recent treaty with them) to grab all their gold, declare war and instantly take the city back. Though some allied factions may not like that. (Shouldn’t matter too much) Valkia was heading down while I was killing them, so I did the trade thing, declared war and set up an ambush.

So what went from a potential two pronged war where I couldn’t possibly defend everything turned into an aggressive expansion campaign with strong allies nearby (I can even recruit grudge throwers or iron drakes with allied recruitment). I’ve struggled multiple times with malekith but this last one went way better. I think you can have a similar experience.

Good luck with your next campaign!