The find a camp and destroy it is just any camp other than ones specifically generated by a quest. You generally need to be a fair distance from a town. I typically found them by going off-road between distant towns. The surest way to find one is to go to the uninhabited side of the map - either East or West depending on your seed - and strike out. You should find one within a day or two. That's dangerous though, as camp difficulty scales with distance from civilization. The safest way to do it would be to find a town with a Tavern and liquor up the locals. You should eventually hear a rumor about a camp.
Edit: Succeeding ambitions has two main benefits. The first is an increase in Renown, which is how well known your company is. The better known you are, the better the missions you get. Once you hit a certain renown, you become reputable enough for the noble houses to hire you. The second is a major morale boost, meaning your guys will fight better for a while after completing one. There are also sometimes other benefits, like the one to craft a warbanner giving you both the warbanner itself and a sash that boosts Resolve.
On your unwinnable mission - are you at odds with any of the noble houses? Because you shouldn't be running into knights on a caravan mission. The worst you should be getting from human opponents is Brigand Raiders, who are typically in rusty chain mail. They can still be rough though. Especially so if they bring archers.
From what I recall, difficulty is mainly scaled three ways - number of bros, number of days, and price of the mission. That last is important - the skulls represent how difficult the game thinks the mission is compared to your squad, but the monetary value is a static valuation of how dangerous it is. More money = more enemies. Caravan missions are also infamous for variable difficulty - sometimes you'll take the trip and the bandit groups along the way get chased off by militia or wander off chasing peasants. Other times, you'll meet not just the bandits the game expects you to run into but an entire additional warband that was naturally spawned on the map. It's possible that's what happened here.
On geists:
Geists have a ridiculously high Ranged Evade stat. Your archers are generally better off firing at something else unless you have overwhelming superiority over the Weidergangers. Even so, if there are a lot of Geists (3+), you may still be best off just spam firing at them hoping to get lucky. Bring bowmen rather than crossbowmen for the extra attacks per round, and equip extra quivers so you can burn through ammo. Other than that, it's melee all the way. An archer will have a 5% chance to hit, while a melee dude will have around 30-40%. In a fight with both zombies and ghosts, the ghosts are THE main target. They will break your formation in two seconds flat. You hold guys in reserve and then charge the ghosts, trying to remove them all before your men break. A single hit will always kill, so bring high accuracy weapons like spears.
If there's more than one or two Geists, I also wouldn't recommend even bothering with the fight unless you have a Sergeant. This is a dude carrying your warbanner, which gives a big boost to Resolve. He'll need to have a high Resolve score himself (preferably 50+), and should have the Rally ability. He leads the charge on the ghosts, and spams Rally to counter the ghostly wails.
Also of note: Geists cannot be Resurrected, even by a Necromancer. You drop them, they stay down. Another reason to prioritize them.