The contract board won't tell you about the lunatic in the ice cave or the trick that turns an enemy giant against its handlers. Here is the knowledge that separates a careful company from a doomed one.
Barbarian Beastmasters whip captive Unholds into fighting for them. Kill the Beastmaster and his Unholds turn hostile to the Barbarians. The flip side: if an Unhold ever ends up on your side (a "fight together" event), it can go wild and switch on you if its handler dies, sometimes as early as turn 2. Treat any Unhold's loyalty as conditional on the Beastmaster living.
Crossbows and Handgonnes equipped in an active weapon slot at the start of battle are pre-loaded. Your crossbowmen and gunners get their first shot off immediately on turn one. Always start the fight with the ranged weapon in hand, not stowed.
Cutting-damage skills that behead (polearm Strike/Reap, 2H sword skills) permanently remove an undead corpse so a Necromancer can't raise it again. In a Necromancer fight, beheading the risen is how you actually make progress instead of fighting the same zombie six times.
A hit to the head does +50% damage to HP but not to armor. That's why flails (direct head targeting) and high-Melee-Skill daggers can kill a target whose armor you'd never break through normally. It's also why a helmet matters more than you'd think.
An unarmed Wiederganger can't hurt you. If you pick up a fallen enemy's weapon (even mid-fight), a risen version of that corpse has nothing to fight with. A cheap way to defang the Undead horde.
A crazed lone Barbarian lives in random ice caves in the North. Beating him requires a heavily armed, heavily armored brother who can win a one-on-one duel, ideally a 2H mace or hammer user with Battle Forged, strong Melee Attack and Defense. Win and you claim his special Head Chopper Orc cleaver and his broken armor (repairable into a strong piece).
Champion enemies (marked with a star, unique gear, extra perks) always drop unique equipment when killed. By the late game you should be hunting champions specifically. They're harder than the rank and file, but the named weapon or armor they drop is often better than anything you can buy.
Weapons from bosses, special camps, and unusual circumstances roll randomized stats. That means a named weapon can be garbage, or it can be far better than any craftable equivalent. Note (v1.5.2.2 era balance): named weapons no longer roll reduced base Fatigue on equip, though reduced Fatigue build-up on skill use is still possible. Judge each named drop on its actual rolled stats, not the name.
Battle Brothers has multiple campaign Origins that change the whole run. A few with notable secrets or playstyles:
Retreat is a valid strategy. The map respawns enemies and contracts; it does not respawn the brother you raised from a Farmhand into a Battle-Forged monster over sixty in-game days. When in doubt, buy this game's most underrated item: time.