I was looking for an almost impossible challenge, and playing Mierce in the "War Without a Breath" actually comes close! Here's my guide on how to do it.
Playing on VH/VH/VH, this campaign's start is extremely difficult! Surprisingly the political VH does add some challenge and intrigue, especially very early in the game when you don't have much money.
The starting position is very dangerous, being surrounded by many enemies who attack immediately. The toughest challenge is having enough food surplus to maintain a decent force. Replenishment will become an issue very quickly, with supplies running out without a food surplus!
The way to solve the food problem is to immediately recruits new generals (2 on the first turn, to make it 4 generals total, and a 5th general by turn 10 or so). You have to fight a lot of battles and quickly (obviously winning), to gain experience and followers. The Forager follower is crucial to maintaining good food production, as a level 5 follower brings +50 food with him! If there is an exception to that rule, it might be the King, with the Quartermaster follower, to reduce army upkeep until your economy gets going (it could take a while!).
Micro-managing everything is very important in the beginning. For example, make sure to fight night battles when you can win with fewer casualties. Maintain just enough loyalty in governess and generals so they don't rebel, exempt provinces from tax if they take up food, etc. You need at least 1 general with level-4 Command skill, to allow you to fight night battles. Raise the tax level immediately to it's highest level to get the tax while you can (before losing all provinces), and get all the highest tier troops you can get, at first you mostly care about food consumption and you should be able to afford these more expensive units.
Strategically, the first thing is to survive, and find a defensible position. I found this position to be holding Tamworthige and Waerincwicum, one of them holding a garrison (exempt it from tax) and the second one a Granary (high-tax it!) for extra food production. They are close to each other, so you can defend both with the same armies when needed. There will be many battles, but the AI seems a bit afraid of long sieges and attacks against fortified cities. As long as you need to deal with many small enemies, it should be fine - but once you face a 2-stacks opponent, things might become too difficult if you're not strong enough!
Once you get comfortable, and this can take a while, probably 30 turns or more, you should obviously look to extend your area of control. There is sort of a "ring", a circle of fortified cities with Tamworthige and Waerincwicum being 2 chains in the ring. Snotingaham, Steanford, Northamtun complete the ring. If you manage that, you will also control the 2 settlements inside this ring (Rocheberie, Ligeraceaster) , which will provide a nice food production addition! Start working on a second general (Ideally with a bonus to unit upkeep) with a level-4 command, to help defend your longer borders. Each fortified city should have a level-1 garrison, and when it is being attacked, sending a 5-10 units stack to reinforce it usually is enough to win.
When you have too many units and this starts being expensive, it is time to start working on the economy. Sack neighboring settlements, and spend the profits in building high income buildings, and start taxing your provinces. You should also regularly send generals to sack neighboring villages and get some income - there's no need to hold them long term, just get the income from sacking. Occupy them and leave them immediately, so the enemy can repair them, and repeat. Don't let your generals get caught in battle though, you don't want to lose your food-makers just yet!
Now that you have a stable kingdom (5 fortified cities, 2 villages), you can look at extending your area of control once again. Now you need to make a long term strategic decision, which direction to go. Due to the raiding fleets of Vikings on the eastern and southern shore of England, it is probably easier to hold the east and push west into Wales (however later face the naval landing from Ireland). Also, many times West Seax has a big territory but they are much weaker than they appear, so that's the direction I ended up taking.
At this point, you should switch from the Forager follower to the Quartermaster follower, to allow for more units and more armies. In this phase, you may also choose a strategic direction based on which enemies are strong, or possibly too strong for you to handle. Whichever direction you choose, be sure to aim for a fortified city, build a garrison there, and be prepared to hold and defend it. Don't worry about negative income too much, keep sending your 1-unit stacks to sack nearby settlements to make up for the missing income. This phase might take a while - I managed to expand this way only by turn 60...
After breaking the brunt of enemy armies, the game will take a turn and become a sack fest. 1 or 2 generals should take the burden of laying siege on fortified cities, replenishing etc. Another general should help defend your territory from invading armies, and the rest (at least 4-5) should go on a sacking rampage. This will both fill your treasury and keep your enemies weak and chasing your armies around. At this point, the game is probably almost over, you cannot really lose anymore, it's just a matter of time to do proper cleanup of the opposition!
I got the Short Conquest victory on turn 114, around turn 80 it was practically all over though, and the rest is nothing special.