This is no doubt the hardest mission in the game period.
If you are not around a fire you will lose health. You will heal health by being around a fire. So as you walk around you are trying to go from fire to fire.
To make things worse the enemy is sending scouts, every time you miss a scout you have to kill off a couple of hussers and the giant enemy army moves forward. To deal with this cycle between your two heroes three abilities and they should go down fast.
Take things slow. You do not need to rush ahead until you have killed a scout. As well you might as well wait on cooldowns of your heroes before moving forward.
You will lose about half of your army after the long walk. This is the first long walk from north to south. For whatever reason a health nerf was made to skirmisher health and they are unable to make the walk. This makes things even harder. It means you have to be spot on with your use of abilities to win this.
It is essential to skip any collectibles. These increase the health of your heroes which should help you but because you spend so much time on them the enemy is sending that many more scouts and that many more hussers.
In the last little push there are many fire pits. You should not stop for all of them, or even most of them. You should only stop when it is absolutely necessary. If you try to get through this quickly you should finish the mission much faster with less scouts getting through.